Also, lately, Nokia contributes to upstream projects more than previously (Bluez, Kernel, Tracker, Mozilla, etc.)

For example, the git repository for Bluez: http://git.kernel.org/?p=bluetooth/bluez.git;a=summary
Yes, there are commits from a guy called Johan Hedberg, guess where he works.

The point here is: contributing to these upstream projects, you're contributing to Maemo.

IMHO it would be a bad idea create garage projects for these projects.

About Nokia projects, there are git repositories for Modest and Hildon too. Hildon people have accepted some patches for Fremantle from Openismus people through of bugs.maemo.org. And I think Modest also have received some patches.

HAM has a git repository too (but there seems be little interest in contributing to this project, else some stuff from this should have been implemented yet).

Yes, I know they aren't going to use radical changes to those, but you always can fix bugs and write some patches (perhaps it's easier they accept a patch than Pidgin devs ;-))
Hopefully, more projects from Nokia use the git.maemo.org system and they are developed more openly in a close future.

Of course, lbt has a right point about co-development and discussion (when the process is in this stage) with the community.

So, is it easy contribute to Maemo? Well, IMHO it's not so difficult like it seems, but it isn't so easy like other projects.

Should Nokia be more open in this topic? Of course. Always it's possible be more open.