This doesn't appear to be documented anywhere in the program of help files, but command-` (below the ESC key on US keyboards) will switch between multiple open iChat windows.
[Editor's note: This also works in the 10.2 Finder ... I honestly can't recall whether it worked in 10.1 or not.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020824081536631