Let me give an example to base the discussion on. I understand the solution must satisfy a very wide range of scenarios however for the sake of keeping this discussion simple lets look at this scenario.
-3 rooms with independent speaker setups
-Family room using A/V receiver as amp and 5.1 speakers
-Porch using un-powered in-wall speakers
-Bedroom using un-powered in-ceiling speakers
-1 Mac with iTunes music collection
-1 iPhone used as a controller
To stream music independently to all three rooms and have the ability to completely control (volume, songs, stream, etc.) each from an iPhone you would need the following.
-1 ZonePlayer 90 for Family room $350
-2 ZonePlayer 120 for Porch and Bedroom (has built in amp) $1,000
-1 Remote (use iPhone app) $0
iPod Touch Solution
-3 iPod Touch $600
-2 Amps (iPod Touch can't power speakers on Porch and Bedroom) $200
-2 iPod Docks (for powering and digital outputs to amp) $60
I know the above solution doesn't completely compare however if it were to be commercialized and the following added it would still be cheaper, allow for more functionality, and you'd have the iPod Touch for more diverse use.
-A dedicated app that synced and controlled all iPod Touch and computers in your setup as well as pull in iTunes library (similar to the app Sonos created)
-A dock with built in amp designed to work with this solution (similar to the ZonePlayer120)
-A dedicated remote to control system if you didn't/couldn't use iPhone (similar to Sonos controller)