Onlydarksets

This looks like an intriguing project.  You can use the iPhone or a Touch to control multi-room audio, at a cost of just $100 per room.  Basically, the Airport Express lets you use iTunes to stream music wirelessly, and iTunes can manage more than one Airport Express.  The iPhone can wirelessly control iTunes, and it can manage the Airport Express through iTunes.

That said, I don’t think this is a true multi-room audio solution, in that it does not appear that you can listen to different songs in different zones simultaneously.

One note – it does not look like you need to hack the WRT54G to support WDS.