OK, so we all know some of the details about AirPort Express by now. Some things that may not be well-known or understood:
All audio is transferred to Express via Apple Lossless Encoding.
You can get digital audio out using a plain old optical cable and a Toslink adapter (the thingy that has a metal prong on one end that plugs into a 3.5mm jack, so digital/analog can share the same jack).
Noting the two previous facts: you can output digital audio to your receiver, as long as it knows how to decode it. So now I can rip my DTS 5.1 audio CDs (44.1/20-bit audio) losslessly and stream it from iTunes through AirPort Express to my receiver. DTS 5.1 audio CDs are just PCM data, like any audio CD. But if you don't get it properly through an decoder, it just sounds like noise. But iTunes + AirTunes + digital connector + DTS-decoding receiver = proper decoding. Of course, don't expect much compression with Apple Lossless Encoding on these files...