RogueAmoeba just linked to Mark Johns' easy little app called Cursorceror. He loves the Ctrl/scroll wheel trick, just as I do (holding Ctrl and using the scroll wheel allows you to zoom in on any part of your desktop-- especially useful for watching Flash movies fullscreen very quickly), and he wanted to get the cursor out of the way quickly, so he wrote Cursorceror to attach hiding the cursor to a hotkey-- make it appear and disappear at will.
The only problem is that he didn't have to do all of that. Turns out that when you're zoomed in (in most applications, including the major browsers), hitting any key at all will automatically hide the cursor for you. So if all you want to do is watch Flash videos fullscreen, just zoom in, tap a key, and watch your videos cursor-free.
Johns' wittily-named app didn't completely go to waste though-- while hiding the cursor is easy most of the time, the keystrokes you hit will still be received by whatever program has focus, and so handing off the task to Cursorceror will keep you from making any mistakes while keying. Also, not all programs will apparently let the keystrokes through (I couldn't get the cursor to hide in iTunes or GraphicConverter in my quick tests), so Cursorceror will come in handly if you want instant hiding no matter what app has focus. It will also hide the cursor after a set time limit, which, as far as I know, OS X won't normally do.