Apple has been pretty quiet about this, but AirTunes now supports sending music to several Airport Expresses simulataneously. Just make sure you grab the 6.3 firmware update for the Airport Express (released on January 3rd) and iTunes 6.0.2. In the Speakers combo box, you can now select the "Multiple Speakers..." entry.
Neither the iTunes nor Airport Express updates referred to this, but from Apple's Airport Express webpage, I did find this tidbit:
Imagine being able to play your iTunes music on whichever speakers in your house you prefer —- or all at once.[robg adds: This tip was submitted by a number of readers, so I just picked one to use. Thanks to readers afgillitt and anonymous (twice) for also sending this one.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060111174136912