This is my identical problem. Been to Genius bar four times and on support twice to no avail. Everyone thinks I'm nuts. This is so cathardic to find this string! I'm not insane! Everytime I tried to use my iPhone to video the problem for proof, it would go away. I realized after waiting a few seconds it often started working (the time it took to get my iphone on video mode!)
I use magic mouse, wireless keyboard and a new MacBook Pro. (However! I had it happen right at the Genius bar in Keynote, trying to Command C, Command V a graphic, when I tried to show the Genius it worked perfectly, like a nice little Gremlin) So that may discount the Magic Mouse wireless keyboard issue to some degree. (I was using my right finger for both arrow nudging and track pad so they were not simultaneous like the helpful description of that problem as well, which I also experience)
I will be showing this string to the Apple support folks to prove I am not nuts.
So most of my noticeable issues are the delete key not working, nudging using the arrow keys goes away, and the command X, C, V combo keystrokes. I am using American English with all updates in place. Apple Store even replaced iWork with their own version on the store hard drive and it still happens. These issues apply to Keynote, Pages and Numbers. So evidently all my third party apps are more compatible with Lion than Apple's native software. Interesting!
PM