It doesn't sound like the IM programs that Yahoo! and MSN provide will look any different in the near future. But they will change.
An announcement from both companies tells us that Microsoft and Yahoo will link IM networks.
I use IM everyday at work. It's a useful way to communicate some ideas quickly, and a great way to share URLs with others. It's not the perfect communication tool, but it has its moments.
This "merger" will mean that people on the IM clients from Yahoo! and from MSN will be able to see and interact with each other.
I use the Yahoo! IM program at work, and communicate with some folks on the MSN network at home. The linking of the two networks makes life a little easier.