Today I committed a bunch of changes (for the future) to SOAP on Slash, in the SOAP plugin. It's an architecture for more easily adding SOAP methods to Slash plugins. You just enter the class name and method in a database table, make the method available in a module (or somesuch) of that classname, and there you go. It handles access control (seclev and subscriber_only, for sites that support subscribers) and provides access to the Slash formkeys mechanism (which, while extraordinary convoluted, works fairly well and is useful).
Eventually I'll port the Journal SOAP stuff over. Everything about the API should stay the same except for the proxy (soap.pl instead of journal.pl), though we can probably hack it to make it work with either one, for awhile anyway.