For those who had the update stall out at 1sec forever and then got the faint nokia logo in an infinite loop: I was in the same situation this morning, then replaced the battery with a fully charged one and viola! I was able to hold down "u", get the usb logo, and use the maemo flasher to reflash the system (with PR1.1-USA) as detailed in various other threads. 5mins later my phone was up and running. I would highly recommend a 2nd battery and battery charger. They are fairly cheap (<$40) online. Not only did it rescue me in this situation, but with my heavy N900 usage, it's helpful to be able to do a mid-afternoon battery swap and immediately be back to full charge.

I suspect the root issue was that while waiting hours for the stalled update, it drained the battery far enough that it was no longer possible to initiate a flash. It appears that those who stalled at 1sec and then ignored the instructions to not remove the phone or power it down during the update fared better because the phone was able to restart while still having enough juice to complete a flash. YMMV.