The wireless software updates released by Apple this week—AirPort 4.1, along with AirPort Express 6.1 and AirPort Extreme Base station 5.5 firmware—don’t contain a multitude of changes. But the fixes that are there should go a long way to resolving the frustrations of a few users who had no workarounds for their troubles.
Apple released AirPort 4.1, which fixes and improves various things including KeySpan's AirTunes remote control support. I guess it shows that this upgrade includes DACP support in AirPort firmware.
AirPort 4.1, combined with the AirPort Express 6.1 firmware upgrade, allows the Express Remote to remotely control iTunes playing through the AirPort Express.