The slippery plastic pads pick up gunk as they always have, but I have yet to find an optical mouse that needed its lens cleaned.
I suppose if you use your mouse on a very dirty surface the recessed lens might build up some lint or something, but you could probably just blow on it to clean it out. I'd think you'd need to go out of your way to get the lens dirty enough to actually affect its performance.
Were it to get dirty, though, I agree that a Q-tip and maybe some rubbing alchol would be the way to go.
Tip: If your optical mouse is skipping, try using a different surface or rotating the grain of your mousepad relative to the mouse and see if that helps--I had a mousepad that if it wasn't "straight" relative to the mouse would skip like crazy, which I first mistook for dirt.