I bought a used airport express base station specifically for this purpose. works pretty flawlessly. Sometimes I get REALLY lazy, and control the computer in my bedroom (that is streaming music to the living room) with the Remote app on my phone.
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with the iOS 4.2 update, i can now stream the sound from my iphone directly to the airport express without using a computer. Basically, that means i can play the music on my iphone through my stereo wirelessly, which is kinda nerdy, and pretty cool at the same time.
To the OP, I really think a used airport express would be your cheapest route. You can find them on ebay for about 50 bucks.