I currently use an Apple Airport express connected to a 2 channel amp. I'm currently using an Audiosource Amp100, but am upgrading to a Sonance or Niles 1230 (12 channel amps) as I add new zones. I only bought the audiosource for testing out the Airplay/Amp/Airport Express system. (It works awesome)
I will be adding an Airport Express for each Zone. Then, I can airplay to any zone using any iOS device. (iphone, iPad, or iTunes).
I have 8 wireless Logitech Squeezebox Radios around the house and love them, got them one or two at a time for $120-150 each. They turned out to be a bit too geeky for consumers though, as the company has killed the SB line and late last year rolled out a similar product with fewer features for about the same price. They can still be found, though I landed on this thread to in search of what was new and worth a look this spring.
What I love about the SB radio is that they can all be synched (or provide separate streams if desired) for audio anywhere in the house, yard, wherever; controllable from any of my devices, and provide easy access to internet audio services, radio or NAS music... all at a cost of half or better than Sonos. RIP Squeezebox Radio.