This is not the mythical revenue model we're waiting to see from Twitter. It can't be. It's just one of those oddball content partnership sites that will look interesting for a while but probably fade away as the Microsoft contract to sponsor it runs down.
There is some cool stuff in here, though, that would be good to see applied back to the Twitter main site. In particular, there's a Digg-like function that lets you vote up a tweet you see on the service, making it stick to the front page longer. This is fortunate, since some of the content that pops up on the site is rather lame--even CEOs put up "This tuna sandwich rocks!" tweets from time to time.
You can also easily follow exec tweeters and reply to posts quickly from the page (as you can on most desktop Twitter apps and on newer services, like Ginx).
To alert users to ExecTweet's existence, Twitter is running an ad for it on the Twitter.com site. This ad spot is being shared, at the moment, with Twittervision and Tweetie as well. TechCrunch reports that these mentions are not paid advertising, but there's no reason they couldn't become paid in the future.
But again, this is not likely Twitter's revenue model, simply because it's so pedestrian. While Twitter could make a few bucks selling these spots (and more money selling even more spots--there's plenty of room on the Twitter pages), Twitter watchers are holding out for CEO Evan Williams to make good on his promise to deliver revenue from corporate users, not just from corporate sponsorships.