Weird. I can try swapping back to my old Linksys router I guess - I don't remember it happening back then, but it's been awhile. I did see it when my Airport Express was a wireless client, but that could still be a DHCP/routing issue, as you point out.
I've also noticed that (as others saw) the...
I know it's an old thread, but someone might still find it useful - or add something. I still have this issue with iTunes 8.x, and it's iTunes related - in that restarting iTunes fixes it, however briefly.
I can also tell you that my "Airtunes" is actually running over gigabit ethernet...