On Apple's side, why is it so difficult to switch between countries in the App Store? It can only be done on the PC, and requires me to enter a credit card for each country every single time I switch the iTunes country. This makes keeping Apps updated from the Japan and US stores incredibly difficult.
As soon as the new Android models are released here (or possibly WinMo 7), I'll be switching away from the iPhone mess.
I know the OP wanted to avoid buying separate amps for each room. I use a pair of compact Fostex powered studio monitors in the bedroom that sound great and plug directly into the Airport Express. This might be an option (although each speaker needs to be individually switched on).
One other consideration relates to interference in the 2.4Ghz wireless band. If you have things like cordless phones and microwave ovens operating, the sound can and will cut out occasionally. Therefore, you may want to look into using the 5.0Ghz A/N band for streaming audio to get the most reliability. However, the 5Ghz band doesn't offer the same range as the 2.4Ghz band, so you'll have to make a decision there.