I've just upgraded from 10.5 to 10.6.3 and all seemed to go OK, except that now no keyboard shortcuts work in any application. The keyboard viewer shows me that the command keys are working fine, and some global shortcuts work fine when I've configured them to (e.g. command-space to produce the Spotlight search box).
But all applications (e.g. Finder, Safari, Chrome, Terminal) ignore any shortcuts I type at them, even if I use the keyboard viewer to type them.
Oh yes, one other strange anomaly: in Chrome, if I type command-C, it brings up the Developer Tools window if the selection is in the main body rather than the URL box.
Another anomaly I've just found: in the iTunes search box, the backspace key and the arrow keys don't have their usual function - they produce blanks. If I copy that text, it seems the key codes are going through uninterpreted (here I typed 'a' followed by backspace, left arrow, right arrow, up arrow, down arrow):
% pbpaste | od -c 0000000 a \b 034 035 036 037 0000006 %
What on earth is going on, and how can I stop it?!