One problem I've been having with Habari is that the xml-rpc mechanism relies on mbstring support being compiled in to PHP. Unfortunately, my host, A Small Orange, for some reason didn't have mbstring support in PHP 5, though they do include it in PHP 4.
Today I put in a support request to get it added. They had it up and running within 2 hours. How cool is that? That's the kind of support you want out of a host, and the lack of it was one of the reasons I left my last host.
Thank you, A Small Orange team!