It is a driver limitation. The good news is that the guy who has been working on getting packet injection working on the N900 has said he fixed some bugs with managed mode and hopefully will be releasing an updated kernel with the fixes soon. I could be mistaken, but if I'm interpreting that correctly along with the ability to do packet injection, we'll also be able to run our N900s as a proper access point rather than using ad-hoc mode to share the connection.

I got an ad-hoc connection with MobileHotspot working on my netbook running Ubuntu 9.10, both with the in-built Atheros wifi and a cheap Ralink usb wifi stick, but I did have to enter the hex key rather than the passphrase (you can see the hex key your n900 using by typing "iwconfig" on the terminal as root).