Well if we assume the n900's behavior for answering a call is like the iPhone (since both I believe operate with proximity sensors) then it shouldn't be problematic.

I also think that given what we know of the n800/n810. Phone calls and alarms generally over-ride the lock. I've had instances where I had the touchscreen and keyboard locked and my alarm went off and I can still press close/snooze. But other then that everything remains locked. Not sure if answering a phone was the same way (I believe it is though given the internet call prompt).

"joikuspot type application to turn the n900 into a wifi hotspot"

Likely doable, wouldn't it be simply be a bridge between the two networking devices? E.g. on my desktop and laptop I can bridge my ethernet connect to my wifi one to my wireless card. Thus giving my 360 internet access.

Unless the wifi in the n900 is only able to receive and not transmit.. (which may be the case) or the access to wireless is somehow encrypted from us.