The title may be misleading, as it probably is detecting it just not doing anything. For holding down any key, it should repeat the action of that key multiple times until you let go of the key (for instance holding down backspace to delete a sentence). It doesn't work for any key anymore; the result I get for holding down a key is the same as pressing a key once.
I don't recall changing any settings related to the keyboard. I'm using Kubuntu 9.10 and I just used this fix
for the dual monitors problem. Maybe that caused it?
But I'm not really interested in what caused it, more so in fixing it. Any ideas?