Since you cannot connect over USB ...

Honestly, I don't see what is stopping someone from developing a Joikuspot -type app for the N900. It would seem to be something that could be accomplished with standard Linux router software... However, as they discovered in this thread, the stock kernel is missing bits necessary for NAT, an integral part of the whole process. But if you're willing to update your kernel to Jebba's custom one, you can get mobile hotspot capability.

I also found a post saying that Joikuspot will support the N900 in the next few months, if you're not willing to do these kind of hacks...