Modifier keys across input devices ignored for first (modified) keystroke

A number of people have seen this problem on the forums: http://ubuntuforums.org/showthread.php?t=1231551

We have USB foot pedals for modifier keys (ctl, alt, shift, etc). After the modifier is held down, it is ignored for the first keystroke of what should be the modified input. For example, holding down shift and pressing the "a" key repeatedly yields "aAAAAA" instead of "AAAAAA".

There is an xev dump on the forum that shows that the modifier event is seen before the first modified keypress.

I've done a number of tests:
1) The error is seen regardless of window manager /desktop environment (tried gnome, xmonad, and none -- just an xterm)
2) The error is NOT seen in a virtual terminal (ctl-alt-f1). Leads me to believe it is a bug in X
3) The error is NOT seen at all in hardy. I also have a jaunty install with X pinned to hardy's version, and the error is not seen there either.
4) The error is seen with any two keyboards. I tested on a karmic laptop between the laptop keyboard and a usb keyboard.

This leads me to believe it is a regression somewhere in X.