Which came first?
a) Airport Express TM disabled on purpose or
b) Time Capsule to address the issue
They better fix this "glitch" to make things right.
I remember using an external attached to the usb of my extreme for Leo during the betas to test Time Machine. I even used a folder inside an Windoze XP machine on the network for it. Both of which worked without any issues.
My point is that Time Capsule has NOT been released, so we don't have any idea whether the Leopard feature will be re-enabled before or after Time Capsule. Which makes it pointless to complain now... at least wait until Feb.
I certainly believe that you had no issues, but that doesn't mean there weren't any. Backup integrity is a serious concern.