For Trillian “basic” (i.e., “free”) users, there’s a new patch available that lets you connect with the Yahoo IM servers. To check and see if you’re current, right click on Trillian in your system tray, go to help | about and see what patch you’re at. If you’re at anything other than “E”, click here to download the new patch. Otherwise you’ll be locked out of Yahoo IM.
If you’re not using Trillian, why not? This is a superior IM client, with the ability to connect to Yahoo, MSN, AOL and ICQ, along with the ability to peruse the many IRC chat rooms that are out there.