So, I know that on Vista you don't see the contents of chat Windows while using Win-Tab (honestly, I meant to go find that thread and post there, but I couldn't be bothered), but there's a bit more to this problem on Win 7.
Windows 7 introduces a new Taskbar (those using the leaked builds will need a security patch hack to see it) which, instead of having text, has little icons representing programs. When you hover over the icon, it will do the little preview like Vista does. However! If you have more than one window open in one application, then a window will appear for both windows!
Unfortunately, the Win-Tab bug applies to this as well. See attachment for an illustration.
Edit: Eek, wrong image!