I just set fire to an duemilanove. Is the smoke toxic?
I've been playing with nixies using a DFRduino. I'm guessing there was a short somewhere and the component to the side of the power jack started smoking. Then caught fire. Whoops.
edit: I forgot the 'other questions'...
Does the location of the fire indicate where the problem is at all? Since it's physically next to the power in is it likely to have originated there?
I believe all smoke, no matter the source, is bad for you. But I would think if you weren't cupping your hands over it and inhaling deeply you'd be OK.
I wasn't huffing it, I was too busy thinking "ooh, pretty!"
next thing you'll see is a CNN story :
"Are kids smoking Arduinos for a cheap buzz ? Film at 11, back to you, Jim..."
I personally think rosin flux has a kind of pleasant odor, but that's just me. Winding varnish on transformers tends to flare quickly, with only a little smoke, but electrolytic caps can go off like a firecracker if you abuse them just right. Then of course you have the various plastic casings of the components, which are probably not good.. and I am betting that inhaling Gallium, Germanium, and other doping agents is not so good an idea either.
That all being said, it's likely to burn where the "weakest link" is, the smallest trace or wire, or component. Pretty much like a fuse in your house.. a fuse is designed to be the weakest link and so it "burns up" before the wiring in your house does. The point at which the burn occurs, like the fusebox being in the basement, is not really dependant on location- it's more important that it's the weakest link. However, you might look right at the point of the burn and see if there's something right at that point.
Worry not.. electronics as a hobby will provide you many opportunities to breathe the unbreathable...
Ouch that sounds expensive.
The nastiest smoke is probably burnt-out LEDs which can have a lot of arsenic and stinks. Burning plastics can give off nasties like cyanide and formaldehyde which cause tissue damage. If it smells bad it probably is bad... Fortunately we're talking about small amounts of smoke here. I hope.
What's annoying me is that I haven't isolated the problem, I don't want a repeat.
That's a real bummer dude... and you'd just gotten it working. Shame. Personally, after I breath in any sort of plastic smoke my lungs tend to become irritated. Luckily I found a 3M particulate mask some construction worker had left in our garage so now I just use that when smoke might be a problem.
Sounds like you took out your 5V regulator.
Might be able to replace it if you have a good soldering iron.
If that's the one, at least lift the 3 legs and see if you have a short between 5V and GND on the rest of the board, indicating other stuff is damaged also.
There are a lot of things you are exposed to in life that are toxic, so put things in perspective. Personally I breathe enough 2nd hand smoke, BBQ fumes, and traffic smog on a daily basis that the least of my concerns is some silicon/plastic/(and a little lead) going into my room's air supply.
As for the source of the fire, I would suggest it gives an idea to start. It isn't necessarily the cause. Here's an unrelated example. Let's say you decorate your house with holiday lights. At some point, a power strip you use caught fire.
Did the fire start because of a fault in the power strip? Or was it because of a faulty light strand somewhere else? Isn't it possible one of the light strands shorted out? For the sake of argument let's say that was the case. The power strip caught fire because it was the weakest link in a catastrophic failure. However, it is not an indicator to what caused the failure, just that one occurred....
2nd example: when a fuse blows; did the fuse fail or did something else in the circuit? (sometimes the fuse fails. thankfully, these are rare.)
Got your wires crossed somewhere with the nixie power supply and the duino supply? 
Upload a pic.
CrossRoads, that's the one. I'll have a go at removing it.
I would upload some pictures but the fake SD card that lives in my laptop's card socket is stuck. I'm not having much luck this week!
Whoa, that's pretty impressive damage!
I think the worst I've done is blow the lid off a TTL part by putting something way higher than 5V on the VCC pin. Blew the top right off, I could see the die cavity.
Putting 100+ vdc to the input of a +5vdc regulator will do that. You probably took out some filter caps around the regulator also before the flames subsided.
About the only thing worth trying is to remove what remains of the regulator from the board and see if the board will operate on USB power. If so then you have a chance of repairing it, if not you got a nice wall hanging conversation piece.
Lefty
I popped the damage part off and now the power light comes on and the PC recognises it, but I get an error when I try upload a sketch.
I'm going to load Blink onto another arduino and swap the ATMegas over, then I can see if the board works at all and if the ATMega will work in a good board.
Edit: Status update: The ATMega in my fried duino is dead. The board works fine with a fresh ATMega and the 5v regulator removed (so USB powered).
So, not a total loss
So, not a total loss
By no means. You have lucked out. The board can still function as a USB only board. You can use it to be a ICP programmer for burning bootloaders onto other chips, etc. I wish I had such luck when some of my projects caught fire. 
The nastiest smoke is probably burnt-out LEDs
No the nastiest smoke and smell in electronics is a burning selenium rectifier. Fortunately you don't get them nowadays. I remember being told I would remember that smell all my life. I don't know about that but over 40 years later I still remember it vividly.
