AFAIK the applications themselves are not developed in the open, but the framework on which they are based is in http://qt.gitorious.org/maemo-6-ui-framework. You can already try it on your N900 by installing the widgetsgallery application or compiling it on your desktop. Should be noted though that this is MeeGo/Harmattan from Nokia we're talking about and may not have much to do with MeeGo proper from Linux Foundation(the code drop without a UI)