I'm another huge fan of the Airport Express, I love iTunes on OSX & Windows, and have a ALAC (apple lossless) archive on a linux box. Run a toslink into the 990, and, like Michael Moore says above, you get a 44.1kHz (native CD) signal, with no goofy re-sampling to 48kHz like nearly all other computer digital audio-outs give you (I'm guessing the 990s USB port resamples to 48kHz, anyone know for sure?).

If you have a WRT54G, you can run the AE in WDS (wireless bridging) with a 3rd party firmware upgrade for the Linksys. WDS is another reason why the Airport Express is such a killer piece of equipment for $129. It lets you use the AE as a completely transparent standalone (no ethernet needed!) wifi range extender. It also functions like as a network print server for a USB printer - or you can run a infrared remote into that USB port to control iTunes from your stereo remote.

Rob