I just noticed this today, I don't think it has been posted. In OS 10.0-10.04 if you changed the desktop pic, it would mirror across two (or more) monitors.
In 10.1 you can now independantly change desktops for each monitor through the Desktop System Prefs (as in OS 9).
[Editor's note: Ah the lucky ones with dual monitors! ;-)]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20011016120413876