One of the major complains of AirPort Express Base Station with AirTunes is you can’t stream to multiple speakers at once. I just noticed that actually you can, this feature is available since iTunes 6.0.2 (undocumentated).

Simply select “Multiple Speakers” and you can stream music to any number of your AirPort Express connected stereos as well as your computer’s speaker.

TUAW noticed this feature 5 months before me. :-)