Mer was/is designed to bring Maemo 5 functionality to the N800/N810 devices since by hardware specs they can't run Maemo 5. It doesn't make sense to have Mer for the N900.

As for running other OSes, within the N900's hardware, you'd need to play in the SDK as GA stated; or wait for movements - like Mer - to prop up once developers have had sufficient time with the Maemo 5 SDK and N900 device.