This wasn't really so much a question of "if," but "when." They were piloting for a portion of 2006 and Sprint launched this past fall. Alltel has also been in the news with their plans. See NYTimes article.
It's exciting from an advertiser perspective to see the carriers enter the market and open up their portals to advertisements. With all of the demographic information that most carriers collect when they register
new subscribers, they offer advertisers the ability to conduct highly targeted advertising. Not all of this capability will be in place on day one. (I was browsing for the new game SIL on the Sprint network and the only ad I saw was Lava Life. That said, I'm not actually a registered subscriber).
The market, however, shows a lot of promise especially when demographic information can be combined with behavioral information (past purchases of mobile content, mobile news and information browsed, search terms, etc.). It has a potential to add a hockey stick to the market for mobile advertising. We've written quite a bit of research on this topic that is included in our new product: Mobile Media and Marketing. One of the remaining – and large challenges – for carriers in both growing and monetizing this market will be maintaining and growing an audience on their own portals – and one that extends beyond the minority downloading applications to their cell phones.