You should never ever leave a lithium rechargeable unattended....that goes for any charger.
But people do forget. With protected cells, the cell protection ought to kick in....
I have forgotten that I've left cells on charge and have not, yet, managed to burn my house down.
Also it's a good idea to have the charger on something that won't burn. Not that I've ever (except deliberately) made a lithium cell burn up, but the possibility is there.
So how long does it usually take to charge these li-ion cells?
It all depends on the cell depletion......i dont run my batts down so they charge up within 20-30 minutes........Your not the only one that has forgotten about left cells in the charger, i have too and im glad nothing has happened.
On a WF-139 a low to flat 18650 takes several hours (up to 6). You don't actually have to watch them every second - I'm not concerned about answering the phone or going to the toilet. The chargers are behind me from where I'm typing and I'll occasionally check to see that nothing is getting too hot.
I always let cells cool down before I charge them - smaller lights can get very hot and charging can farther heat the cells so I let them cool off before charging. But I do own quite a lot of 18650s.
I like to rotate my cells once a week and store them without putting them back on the charger. I got most of my 18650's from old laptops, all it takes is a little time and it saves you money in the process. Another good device to have in handy is a multi meter but since you used to be in RC's i know you have one.
Actually I don't have a multi-meter. Since my rc's were nitro, there wasn't hardly any electrical/electronics troubleshooting.
So do I need a multi-meter?
They are certainly incredibly useful. But not essential.
You don't need a fancy one and shouldn't have to spend more than $10 on one. This one http://www.dealextreme.com/details.dx/sku.594 will do the job just fine but you should be able to find one around you for much the same or less money.
I'll write something up soon about meters and their uses.
Is a multi-meter a must if I switch to li-ions? I'm thinking about getting the Pila IBCT 4-stage Charger since it seems to be one of the more popular and reliable li-ion chargers out there. Good choice?
I'd recommend it. It is a good idea to keep an eye on voltages. It is a bad idea to try to charge a lithium ion cell that drops below about 2.5V. If it recovers by itself after a while, then charge it, if it doesn't, discharge it as completely as it'll let you and toss it.
If you are going to get lithium-ion batteries, you should get a multimeter so that you can monitor the voltage of the batteries as you use them (and try to figure out when you hit 3.6V, which is about time to recharge) and also measure them off of the charger to make sure your charger is behaving correctly. I got this one and am very happy with it, especially for the price:
http://www.amazon.com/Sinometer-DT830B-20-Range-Battery-Tester/dp/B001T3Y8XW/
You can certainly spend a lot more for a meter, however I say buy cheap and if you use it enough that you break it, then get a better one next time. If you never wind up needing it, then you haven't lost as much. The one above can also be used to measure amp draw on a light, too, but only works if you use the 10A fuseless setting.
That's cheaper than any of the DX offerings. Looks perfectly adequate for the job. I've had many meters that didn't cost much (I tend to give them away) and only bought a more expensive one (well used) on eBay when I had a lot of measuring to do and accuracy mattered. I still use two other very inexpensive meters on a regular basis. But then in a past life I was a science teacher.
Thanks again guys! The thing I love most about BLF is that you have really friendly and knowledgeable people that take the time to answer questions no matter how stupid they may sound. I just asked a simple question on CPF last night (after not posting for a month) and of course they just ignore it. So I just want to thank you guys again.
YOU GUYS ROCK!!! 
You just have to love it here..........i say go for the pila as well since you cant go wrong with the charger......but you still have to tend to the cells, all you need is just one that goes rogue and i dont care who makes the cells.
There is no such thing as a stupid question. Except the one you didn't ask.
If you don't know, ASK!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
I used to be a teacher 25 years ago and my opinion now is still the same. Yes, there are some questions that it is a bore to answer, but...
If the person asking has no reason to know already, it is NOT a stupid question. Once you've explained it a few hundred times it may feel different but nobody's written an FAQ here yet. Maybe one day I will do so, but that is way too much effort.
Meanwhile, please keep on asking. There is no better way to make sure you understand what you are talking about than trying to explain it to someone else. If you don't understand it yourself you will have a hard time explaining it.
So keep on asking. We've all had to learn this stuff - I knew none of it till I bought my first light.
This was about 6 years ago. Some of what I go on about now was informed by my first degree (in science), but less of it than you might think.
We all have to learn every day so never feel embarrassed about what you don't know. There is an amazing amount I'll never know - you know some of it that I never will.
So keep on asking. That way you learn stuff. not asking means you don't.
I know which I prefer and it isn't not asking.
I can still remember that day in fact........
I went ahead and ordered my first li-ion batteries and charger with your help of course.
Here's what I got:
UltraFire WF-139 3.7V 14500/17500/18500/17670/18650 Battery Charger
TrustFire Protected 3.7V 900mAh 14500 Lithium Battery (2-Pack)
TrustFire Protected 14500 3.7V 900mAh Rechargeable Lithium Batteries (2-Pack)
TrustFire 16340 880mAh 3.7V Rechargeable Li-Ion Batteries (2-Pack)
TrustFire Protected 18650 3.7V True 2400mAh Rechargeable Lithium Batteries (2-Pack)
Then all of a sudden, I blacked-out 
and when I woke up this flashlight was part of my order:
SKYRAY S-R5 Cree XPE-R4 5-Mode 320-Lumen Memory LED Flashlight (1*18650/2*16340)
This flashlight is Al's fault since he put the idea in my head earlier this morning. LOL
Hey, chatting with you this morning you told me that the light looked ugly...and what changed your mind about getting the 139 over the pila.
Like i said, "I blacked-out". Maybe I was possessed or something. lol
Lets not go there......i can find someone over there to rid you of that. lol