There is now little work left to be done before 0.5 is declared final, so today the first official beta release was seeded to testers (see the post to the beta testers mailing list). There are still some minor issues to be fixed during the beta period, but the software is quite usable in its current form.
The 31 bullet points in the full change list belie the huge amount of under-the-hood changes that have taken place since the last release. This is basically a total rewrite; painful but necessary to allow for future growth.
I am hoping that most of the major architectural overhauls are complete so I should be able to add features quite quickly from here on. The current plan is to hit 1.0 by the time Apple releases Leopard in October 2007.