That question has been asked multiple times.

The answer is that there's no official answer yet. It is too early for Nokia to commit to anything. However, expectations are that Maemo 6 devices will bring a smaller processing power jump than is occuring between OS2008 devices and Maemo 5 devices. Therefore, it appears probable that Maemo 6 will work on Maemo 5 devices.

If not, there's always Mer, which will be developing alongside Harmattan.

Buy (or not) based up on what the N900 will give you now though, not what might happen in a year or so.