I had a user on IRC run in to the same issue.
The person had upgraded from Kubuntu Gutsy to Kubuntu-KDE4. (He or she wasn't running KDE4 packages in Gutsy)
The user also had a USB keyboard by Microsoft, but I don't think that using any USB keyboard causes this bug, since I have a USB keyboard myself.
The user was able to work around the issue by changing the keybinding for krunner from "Alt+F2 (default)" to Alt+F9, and then changing it back to Alt+F2.
I had a user on IRC run in to the same issue.
The person had upgraded from Kubuntu Gutsy to Kubuntu-KDE4. (He or she wasn't running KDE4 packages in Gutsy)
The user also had a USB keyboard by Microsoft, but I don't think that using any USB keyboard causes this bug, since I have a USB keyboard myself.
The user was able to work around the issue by changing the keybinding for krunner from "Alt+F2 (default)" to Alt+F9, and then changing it back to Alt+F2.