If you want to do it on the cheap, check out WiFi2HiFi, which we've mentioned a couple times in the past. This US$3.99 app runs on your iPhone, and you also must download a free app for your Mac. For those readers who use Windows, there's also an app from that platform.
Install the desktop app, then start the iOS app. The devices will pair over your WiFi network, and any audio coming from your Mac will get wirelessly routed to your iPhone, iPad or iPod touch. If your iOS is in a dock that connects to your music system you're in business. Your iPhone is acting as a lining device to all the audio on your Mac.
Sure, there are other ways to do this, and if you have an iPhone, tying it up for this purpose might not be too great if you want to use your phone. On the other hand, it might be a good use for an older iPhone or iPod touch. There is about a 3 second delay in music, but buffering reduces any potential dropouts. The app has a visual indicator of the state of the buffered audio with colored lights.
Support is good, with an extensive FAQ both on the developer website and the iOS app.
In the course of my testing, as I was switching outputs around, I lost the stream but restarting the iOS app brought it back. Setup is easy, and if you want your music collection on your stereo system this is an inexpensive solution. The app runs on iOS 3.0 or greater, and supports the iPhone, iPod touch and iPad.