I like DaveP1's suggestion in general, namely keeping the hardware releases "bracketed" in with the related OS releases. If/when other devices come out that use Maemo 5, they can be placed within the Maemo 5 section.

I guess things will get tricky if/when the N900 is upgradeable to Maemo 6, but things can be reorganised if that happens. Until then, at the moment there's pretty much a one-to-one mapping between hardware and OS version.