I’m writing this post using the Windows Live Writer beta and it seems to work pretty well. You have to enable the XML-RPC functionality for WordPress to be able to use the software on it (basically chmod +x the xmlrpc.php file) but after that it works like a charm.
The only problem so far really is that WordPress doesn’t let you edit any of your static “page” content through xmlrpc.php but maybe eventually they’ll do that.