In the US, there has been a meteoric rise in the number of video streams delivered to cell phones. A variety of factors are driving this:
1. Improvements in the multimedia capabilities of the "average" handset in use in the US
2. Expansion in the number and quality of mobile web sites
3. Emergence of affordable multimedia data plans
4. Marketing of multimedia centric mobile services like Boost
ABI Research projects that 462 Million consumers will subscribe to mobile TV services by 2012.
Statsistics demonstrate the small but rapidly growing nature of this market. For the latest data, I recommend you visit RCRNews.com -- the web site is an absolutely excellent resource for mobile data of all sorts, including mobile.
Here's a somewhat dated view of mobile video users:
One of the best pieces of news for advertisers is that most people expect ad supported mobile video will be the hottest growth area -- a majority of consumers consistently state that they are happy to watch ads in exchange for free content.