I've had some problems with posting to my WPMu install using standalone editors. You get the error "XML-RPC server accepts POST requests only". That seems to be a problem with PHP 5.2.2, NOT WPMu.
There is a workaround here; http://bugs.php.net/bug.php?id=41293
I placed this in xmlrpc.php at the start, and this solved the problem for me.
HTH
Regard,
Rune
