The 2 biggest missing pieces of the Neo900 software stack from a Maemo point of view are A.The pulseaudio blobs (pulseaudio-modules-nokia-*) which need to be reverse engineered and cloned (work has been done on them but it isn't finished as far as I know) and B.A replacement for the cellular services daemon that talks the same dbus interfaces as the stock cellular services daemon (it will likely be FAR easier to reverse engineer the dbus interfaces and replace the daemon than it will to reverse engineer the ISI stuff and write a translation layer).

If I had any spare time (I dont) I would try and do more to contribute to this work (cellular services daemon specifically)