My N900 should be arriving in the next few days, but there is somthing that has been nagging me, i've read that the process for putting a new firmware (updating Maemo to a new version, or putting in one of those versions that handles the processor clocking better for example) involves hooking the N900 to a computer and also having to backup data and loosing installed programs. Can i instead have it work like how updates to Ubuntu work on my desktop machine, the OS itself downloads the updates and then installs them, without needing any help from other computers (i mean, besides the router and the stuff in internet etc), and without erasing any data etc ?

Yes, that is the default method of installing updates.

what about non-official versions? Can i do it like that as well?

good luck with that one - I've never had any luck trying to install over the air updates, and had to pretty much wipe my phone each time.

If you want a better chance of being able to do this, don't use any apps from the extras catalogue. I've avoided the extras and still find my root folder becoming too full to install new firmware.

sucks big time. I Suggest you open the phone and update firmware right away