Yes, you are free to port Ubuntu, FreeBSD, Android, Solaris, Haiku, or whatever to Nokia N900. The bootloader can execute a payload from a different partition. I don't see why that'd be different from Nokia N810 for which alternative OS are available, such as Easy Debian, Android, Mer, ... I think Mer folks continue to develop on Mer while OTOH I do not expect many people interested in alternative firmware before the stock one is deemed to be limiting factor. So, right now, you can expect people fiddling with the Maemo 5 OS. Its what comes standard with device.