I tested this on a couple of machines yesterday. You need Yahoo Messenger Beta 8 to communicate with Windows Live Messenger Beta 8 users. Older versions of either don't seem to be able to handle the cross network communications.
Now this is all on a PC. I have no doubt Blackberry versions of any software that uses either of the two networks will need to be updated to talk to the other networks.
Of course, if you're using one of the multi-network IM clients (I'm using IM+), this is completely irrelevant!