Sorry, I couldn't find a definitive answer on this...

I currently use the GoogleTalk client and Jabber to connect my BB to MSN Messenger, but with all the new connectivity between MS and Yahoo and the latest versions of their clients, there appears to be another way, at least on the PC.

So has anyone yet got the BB Yahoo Messenger client to talk with MSN Messenger users, or do we have to wait for another version of the BB Yahoo client?

I guess that's a 'no', then

The interoperability between Yahoo and MSN is built into the applications and is purely beta and only on the latest versions of each. I doubt we'll see such capabilities available from the RIM clients. Best we can hope for is a multi-network client straight from RIM. I have a unsubstantiated hunch that we'll see that at some point in the next year or so.