In Snow Leopard I used to use Option + cursors to move around spaces (not Command - this was because I liked Command + cursors for 'Home' and 'End').
Therefore I also used Option + 1, Option + 2 etc to jump to spaces. Therefore my Option + 3 never gave the hash symbol, it jumped to space 3. It wasn't an ideal solution, but I didn't use the hash key much.
Now, Lion seems to have kept (some) of this preference. Option + left/right cursor still moved between desktops (spaces), but Option + 3 did nothing (no hash, no space movement).
I'd like to restore Option + 3 to do the hash but I'm not sure how; I've deleted com.apple.spaces.plist from my library after finding a similar question on here, and rebooted. It has removed the Option + cursor functionality but not restored Option + 3 to be the hash key.
To summarise, Option + 3 now does nothing, and I'd like it to do what it should do. When I look and the keyboard viewer and press Option, it shows the hash symbol on the '3' key, but when I press it nothing happens. Keys 1, 2, and 4 don't work with Option, either (I previously had 4 spaces in Snow Leopard, so I'm sure this is the cause).