Wow that was technical. Think we have a furnace guy here on the forums.
I'm no furnace guy, but I had the same problem, TWICE. And all you guys talk about darwin awards is a good point. The more I look back, the worse I feel. One wrong move and I could have been toast, literally.
But I'm a DIY guy so I had to try, and it worked both times. So here goes, very non technical description of what I did.
First time, draft inducer blower (heh, picked that up from kazoo) would turn on, glow plug would light up, gas would turn, and furnace would function for about 10 seconds, then just turn off. It would try this about ever 2 minutes. It ended up being the flame monitor (maybe the cad cell eye in kazoo's post??). It is a small cylinder that is up in one of the flames. It was dirty. They said to clean it off with an emery cloth, so I used one of my wifes emery boards. Worked like a champ.
Second time, draft inducer blower would fire up, but glow plug would never turn on. Took it out and realized that it had a crack in it, got a new one from a local utility place, installed it, and everything went back to working.
This thing breaks down one more time, I'm calling a pro.