Is it possible for iTunes to send a single AirTunes stream to multiple Airport Express base stations?
It is possible...
Yes, you need to be running Airport Express Firmware 6.3 and iTunes 6.0.2 or later.
From the Airport Express FAQ
Question: Can iTunes send a single AirTunes stream to multiple AirPort Express stations?
Answer: Yes. With iTunes 6.0.2 or later, you can send an AirTunes stream to multiple remote AirPort Express units, provided that each AirPort Express has the latest AirPort firmware 6.3. The maximum number of remote AirPort Express units is three to six in typical conditions. The number that works for you will depend on your environmental conditions (such as building composition and local radio interference), distance to the remote AirPort Express units, and available network capacity relative to your other usage.