Volvo and Intuit have used Twitter in their banner ads. VW had a good campaign analyzing your Tweets to recommend a car. Some are trying to show those Tweets in the banners themselves, allowing people to create the content themselves (after someone screens it, of course).
Yesterday I stumbled across another variation from the Harry Potter and the Half Blood Prince U.K. site. The entire site focuses on getting you to perform some magic on one of your followers' Twitter page, while it sends out a Tweet that you're doing so.
- The site takes some proven technology (page takeovers) and performs them on your personal page.
- While it taps into Twitter to broadcast the message that you're using the campaign, only one person at a time can actually SEE what you've done.
We're still in the infancy of Twitter advertising. Although, really, the best Twitter advertising happens in the Tweets themselves: reviews, recommendations, rants and shout outs.