Other than sending it to Nokia, I don't know how to actually FIX the problem. However, if you're ok leaving your phone on all the time, it's possible to turn it on, assuming you're in the same situation I'm in.
I removed the front and back covers, then pulled up the top piece that covers the power button. The button itself had fallen off and was bouncing around inside, it's about 2mm by 1mm rectangular, with a little black switch on a long side. I'm not sure how to correctly reattach it, so I just left it out.
Once you have the emtpy area below where the orange button would go, there are two contacts just on the inside, against the front part of the phone. Simply short those two contacts, with a battery inside, of course, and your phone should come right on. You can then reassemble the covers around it.
Kurt
January 2008