Mambo's newest -- ver. 4.6.2 was released about 2 weeks ago. I've got a couple of them up and so far it seems a job well done. Very few issues on their forums, and all minor.
The big change for 4.6.2 being full internationalization for both content and administrator. They've been working on this for a while -- was supposed to have been out in 4.6.0 -- and now its here and it works well.
With Joomla 1.5 beta looking so good, this is sort of a redundant post but at least it shows Mambo is still breathing. I checked out the codebase and to me it's got more and more spaghetti like since I last looked and the level of commits seem very indifferent.