Just to say that it is entirely unproblematic to run Gentoo in a chroot jail on the N900 as per these instructions

pro:

• (for me) a well-known distribution
• newest software, same as on my big machine
• no version-skew problems, except possibly the ancient kernel, no problems so far though
• N900 system untouched, all the new stuff runs in the chroot jail
• communication with N900's own software via dbus
• X works, using N900's own window manager.
• backup is easy (just tar-gzip the jail)

con:

• builds are slow, distcc is a must
• Programs can look funny (no Hildon)

If anyone is interested I can post some more details.