BlackBerry App World will see carrier billing by 2010. Jim Balsillies, RIM’s co-CEO, made the announcement in a keynote yesterday at RIM’s BlackBerry’s Developer Conference.

Carrier billing would make it easier for BlackBerry customers to purchase applications on their phones. As it stands, BlackBerry users have to enter PayPal credentials, which some say inhibits spontaneous purchases.

Meanwhile, RIM made a handful of announcements aimed at fostering development for its app store. The company introduce the new BlackBerry Theme Studio, for customization of the BlackBerry UI. Also, the company said it would include a new advertising service for developers, which will make it easier to integrate ads with applications.

While RIM has made no official announcements as to the total inventory of BlackBerry App World, the number is believed to be quite low relative to Apple’s 100,000 applications at the App Store. Current estimates put the number at around 2500 available apps on BlackBerry App World.

Today marks the last day of RIM’s Second Annual BlackBerry Developer Conference in San Francisco. The conference began on Monday.