Intial instructions have surfaced showing the steps that need to be taken when installing a dual boot setup.
This feature is currently experimental and intended for experienced users only. Several commands here are dangerous if executed incorrectly and may damage your Maemo installation or your N900. If you are unfamiliar with image creation, the flasher and MeeGo you are strongly advised not to proceed any further.
These are the usual disclaimers, but in this instance I would REALLY take heed.
Onwards and upwards, for the ones that I haven't scared from performing the dual setup, please follow the full instructions located at MeeGo-Wiki ARM/N900/Install/U-Boot from scratch
Best of luck and please let us know how you get on.