Though I did not click the ad, the HTML source of the Adsense iFrame does reveal lot of interesting information about this Adsense Ad:
» The Adsense gadget has an embedded Youtube player that serves the four videos of GMail Theater Ads directly from Youtube.
» The YouTube player is 250x200 pixels while the Adsense Ad size is 300x250 pixels.
» The Adsense Flash widget is actually served by Lab Pixies, a company that also creates gadgets or widgets for personalized pages like PageFlakes, Google Personalized Homepage and Microsoft Live.com.
» Though the images in the ad are served by LabPixies, they are redirected via gmodules.com/ - a Google owned site.
» Clicking the Tips Tab on the Ad will show the following tips:
"Is any chat really complete without a flying monkey? Spice up your messages with these Gmail emoticons.",It is unclear whether these are CPM or CPC ads since they mix Flash video with text, something that we haven't seen before.
"You can make any endless email thread invisible, but just a quick search away if you ever need to catch up.",
"Choose any photo as your Gmail pic and it will pop up next to your name on people's inbox and Contact lists.",
"Keep your social life hopping by easily adding event info from your Gmail messages to your Google Calendar.",
"To find all emails from a certain someone, type from: and their first or last name into the Gmail search box.
Update: The performance of these Youtube Adsense Ads can be easily guess from the links feature of Youtube. Full story.