Right now, Nokia controls Maemo 5. But, OP, are you aware that there is an alternate version of Maemo 5 called Mer that will be community-controlled? This is amazing to me, and it's possible because so much of Maemo 5 is open-source. Mer is still being developed, but it is able to install apps from the Ubuntu repositories, which opens up your options pretty wide. When your N900 turns into a legacy device, you'll be glad for this policy b/c it will be able to extend the lifetime of your device.