New from Adobe!
As of flash player 9,0,28 you can do fullscreen in the browser.....

this requires some hand coding of the HTML file to get it to work (thanks Wilbert). This is really cool and while not for useful for most sites as flash player penetration is still low for the most recent release, it will get there fast.

Here's a little test I did
http://bretlanius.com/flash/lab/fstest.html