You probably like Debian for its (mostly) well-working package management. And although Maemo uses the same tools, it's a totally different story here. Debian's ease and stability doesn't come with the tools (alone). It comes with the strict policies of when a new package is accepted and when not.
Fremantle is a fork of Debian from a time when Squeeze was testing. It's essentially a Lenny/Squeeze mix, with some parts even dating back to Etch, which were apparently carried over from an earlier Maemo version.
Are you actually interested in a new OS on the N900 phone or are you just unsatisfied with userland development options of the N900 as a pocket computer?