I came across a fix using a little program called Bytecontroller:
http://www.macupdate.com/info.php/id/15170 . It's designed to control itunes from the menubar.
In its preferences you can set hotkeys. Set F4 (i.e., Fn+F4, remember "volume down" is the default, you have the hold Fn to get the F4 key) to be decrease, and F5 to be increase volume. Newer keyboards have different locations for volume up and down, I believe.
To control Airtunes speakers, you then just hit Fn+F4 or Fn+F5. Hitting F4 or F5 without holding Fn still controls the system volume. You don't even have to alt-tab to itunes, it works from whatever program has the focus.
Hope this helps - I'm happy with it.
~Ste