Techdirt has a post telling how Yahoo is going to continuously change their instant messaging protocols to make the instant messaging client, Trillian, incompatible with their network. I have used the free version of Trillian for about 3 years; it allows you to make connections to all the popular instant messaging services -- AOL, Yahoo, IRC, MSN -- simultaneously and with multiple distinct accounts. The best part is that you don't have to see any of the ads or crap that goes along with using the free versions from the respective companies. Furthermore, there is an option in Trillian to enable 128 bit encryption over AIM, which I have used and have to say works very well.
I guess the only upside for me personally is that I don't have a Yahoo ID, so I don't care so much, but it seems a little silly for Yahoo to be so concerned about another client accessing their "free" service.