It could be both... It could be hardwrae failure while booting.. or if it boots then barely goes to desktop an reboots that means it's software and there's a fix for it on this forum... I suggest you search TMO and try the solutions that the people came up with before contacting Nokia Care, Also there's no new firmware updates after the one you mentioned right now, but soon PR1.2 will be released as it's now in the RC state... PR1.2 is a BIG update and very much anticipated as you might've noticed by now...

It runs Linux, not Windows'95. Any system crash should be a kernel or hardware bug. A bad application should just crash itself, or drain the battery, make the system slow.
Which apps have done so much damage that the phone crashes?

However the N900 tends to reboot itself for reasons other than a system crash. If it has trouble on boot-up, (e.g. bad config file) it will just reboot and have the same problem again, instead of dropping into text mode so I can fix it :-(

I've had my phone since launch day in November and haven't once experienced this problem. Devel apps have caused these reboots for many people. Despite warnings, people mess around with the devel and get burnt.

I have also not experienced this bug, or alot of them that you all are. I wonder if its because I bought mine 4 months after it droped...

Not that this adds any value but I have never experienced this bug either, I dont install from devel but have quite a few apps from extras-testing on my n900... Never seen a single reboot I did not initiate myself!