Apple TV, iTunes, and possibly the Airport Express support a daap remote control protocol (edit: called DACP).
it is not as nice as the Roku's Control Protocol, but is nice as we can use iTunes, winamp, iPhone, and others to control the devices.
The iBridge project is writing a DACP to RPC translator. Big thread and lots of comments/ excitement.
Is there any chance the SoundBridge will support this protocol natively? Would it be available for the 500?
Oh and I think Roku rocks!