I just noticed that for animated image ads that are being run on Adsense Google provides you with a little button to close the ad.

http://www.quikimg.com/uploads/googads.JPG

This is perfect for when one of those flashy ads is annoying you, now you can just close it.

The close button is kinda small, don't you think?

Wonder what the "accidentally-missed-the-close-and-clicked-through" rate is?

There's no reason why they couldn't put the "X" near the "Ads by Goooooooogle" instead. It could close all ads in that ad unit.

Is this really Google's "x", or is it possible the advertiser included this to trap people with a non-precise mouse onto their site... and did it really close the ad, and what was displayed afterwards?

Yes, it general behavior, I saw it at dozen sites. The ad just disappear.

Interesting. I always disable animated GIFs, so they might show it only for those ads. But the X should clearly be bigger.

I remember a Flash ad (not from Google) in which I had to hunt a small close message that moved around everytime the mouse was close.

<< This is perfect for when one of those flashy ads is annoying you, now you can just close it. >>

you can always use Flashblock :)
http://flashblock.mozdev.org/