and, Debian or Ubuntu works in N900 slower than Maemo.
I think, really comfort of use Neo900 with Debian is very low.

I think, Ubuntu Mobile can be good choice, but it is'nt ported to Android keyboard smartphones

May be, Gentoo will be work fine on Neo900, but it is need more time to install and optimize software(more dificult than x86, I use gentoo in laptop). It will be intresting, but not sure I am have at this time.