Unless your amplifier is some sort of smart computer you need an anologous way to transport the sound. A/V links are quite pricey, they have a transmitter and a receiver (can also send video). The cheaper solution is radio transmitters that will send to any FM frequency, so that you can play it from any radio inside your house (well as far as the signals go).

there isn't a bluetooth configuration for wireless stereo yet -- but on another note, check out Salling Software which lets you control iTunes from a Bluetooth enabled phone (plus lots of other stuff)

http://www.blueserker.com

If you want to go the cheap hacking route, Radio Shack $20 FM tramsmitters work pretty well. I rigged one up with a 3v wallwart so I don't have to deal with batteries. Now I transmit to various boomboxes I have collected through the years. Each room has local volume control so I can have music everywhere I go without one room blasting.

It depends on your need for quaility vs cost. Obviously, this isn't the audiophile solution. It's cheap and it works well enough until you decide you are willing to lay out the money for something better.

Not the cheapest solution (a few hundred), but a really nice one if your stereo is near your TV and you've got WiFi+broadband: Tivo + home media option w/TV wired to Stereo. The Tivo sees all your iTunes playlists and lets you pick and choose music from your TV screen. Best part: you can load internet radio stations into an iTunes playlist, which will let you listen to pretty much any radio station that's got an online feed, through your stereo.

you're looking for the squeezebox. lets you stream MP3s to your stereo.

Yup, Squeezebox is what you want. Will also stream aiff's, oggs, and other things.

I write more about it here giving advantages, disadvantages.