The number of Twitter users accessing the service via mobile has spiked 62 percent since mid-April, thanks to the release of official Twitter applications for iPhone, Android and BlackBerry phones.
During this time frame, the number of users who signed up for Twitter account on a mobile phone has increase to 16 percent, up from 5 percent.
Twitter claims that 7% of its users access the service on a Blackberry, while Apple iPhone accounts for 8%. Twitter’s mobile site remains the most popular, used by 14% of its users.
Overall, some 46 percent of Twitter users are active on mobile. There are 145 million registered users on Twitter.
http://blog.twitter.com/2010/09/evolving-ecosystem.html