From iTunes, I can stream to any and all of my three Airport Expresses (or should that be Airports Express?!!) simultaneously and control volumes independently too.
I can also do so from direct my iPhone (as well as control streaming from my laptop's iTunes library using the free "iTunes Remote" app).
AND with the new iOS (4.3?) and iTunes10.2 (still downloading) you can stream from iTunes direct to any iPod/iPad/iPhone/AppleTV, so if you have a stereo with an iPod dock, you could effectively use your iPod as another Airport Express and stream to that too. Given that am sure many more people have an iPod dock than an AE, it could really increase the number of people who get into wireless streaming...
The update allows you to stream movies too, which could be great - although am wondering how long it will be before the OH drops iPhone in bath while streaming some chick flick! ;-)