Nokia already offers Meego for N900 users. You can install the other OS on on your MicroSDHC card and easily multiboot to whichever you want, or you can flash your device with it. However, what we have now is very early implementation and most of the userspace functionality (not to mention graphical gimmicks) seem to be missing. It also runs very slowly on the N900, so I suspect that new hardware will be necessary to make it run properly.
The CPU and other specs of N900 are perfectly fine but 256 MB RAM simply isn't enough for modern desktop Linux which is what Maemo and Meego are.