Mystery icon.... LOTS OF POSSIBILITIES
The mystery icon is...(and remember

use the word "REVOLUTIONARY" in reference to the iPhone and I think that means a lot more than what they have unveiled)
1) YouTube? (Well, maybe, but since they already announced that on the AppleTV that wouldn't be a big deal, and couldn't you access that via Safari on the iPhone anyway? Not Revolutionary, although I do think YouTube is cool and fun.)
2) GPS? (That would be GREAT! And that would be revolutionary. Can that fit into the iPhone case they are showing? I haven't a clue.)
3) Video iChat? (I think there is a camera hidden behind that screen, we saw something like that in an earlier patent release. That would be revolutionary! I know it sounds crazy, but go back over MacRumors archive and find that patent prior to touchscreen announcement and you may understand why I say this. From iPhone to iPhone, and iPhone to Mac. Pretty darn cool. Okay...a longshot, but I am hoping.)
4) Screen Sharing on your iPhone with you Mac & AppleTV? (Imagine being able to access your computer screen on your iPhone and run any app, view ANY movie, etc. That would be huge. Possible? Maybe. Leopard will allow iChat users to screen share. Why not iPhone. It would clear up a lot of memory questions. YouTube "magic" could be used to get all of your videos and TV shows to your iPhone.
5).Mac? (Steve has recently mentioned "making up for lost time with the .Mac community" and having some big plans for .Mac users. What could that spell for the iPhone. Anything? Nothing?
6) TV? (Will we be watching live television or TV with a slight delay via some Google/YouTube mystery application? I REALLY think this is a possibility. Google acquires YouTube for Billions, Google Teams up with Apple,...other phones are going to offer TV on a teeny-little screen. This seems very revolutionary and a good fit.)
7) Lightsaber? (As much as I would love to see it....no, I don't think this will be a secret feature.)
These are my best guesses.
