Craig Latta's project, Spoon, does (as far as I know) remote object invocation. http://www.netjam.org/spoon. You get a tiny image for free.
That's right. Spoon's remote messaging system supports arbitrarily nested sends (local object A sends to remote object B which must send another message back to A before answering A's first message, etc.). It also supports messages whose parameters live on multiple different machines. Finally, the system works across multiple processes on the same or multiple machines, and the contexts of a single process can span machines (as can happen during remote debugging).
In my use so far, it has provided a transparent illusion of a co-located set of objects across multiple machines. But I was hesitant to respond to Rich's original request, because Spoon isn't yet directly interoperable with mainstream Squeak releases (we're working on it).
thanks,
-C
spoon@lists.squeakfoundation.org