I have been having the same problem primarily with KMail. Here's what I found:
I found that deleting ~/.kde/share/config/kmailrc resolved the issue, but after some use the problem began to reappear. A diff of a problematic kmailrc against a pristine copy turned up (in the bad rc): [code:1][Composer]
Height 768=705
Width 1024=1025
--snip--[/code:1]
The Width parameter specifies 1025 against a screen width of 1024. Why this is happening, or why it causes the wierd kb repeat, I havn't yet discovered.
To fix, I simply reduce the composer window size, exit and restart KMail, and it's all better. It's no big deal keeping the composer window small, since line wraps negate the benefit of a maximized window. I haven't had a problem with any other apps, but you might check for a similar issue.