The release is limited for now. Developers must use Visual Studio 2010 Professional or higher. Visual Studio 2010 Express for Windows Phone is not supported.
In addition, developers can only use Visual Basic for Silverlight WP7 apps. That means that Visual Basic developers can't build WP7 apps in the XNA Framework, which is Microsoft's gaming runtime environment.
In a blog post about the Visual Basic add-on, Microsoft's Larry Lieberman called the announcement an initial release, indicating that the company could in the future include support for the XNA Framework.
Phones running WP7 hit the market in early November. Users could choose from more than 1,500 applications initially. The store has grown to more than 2,700 apps now, according to a website that lists and tallies the apps.
Opening WP7 app development to Visual Basic developers could help Microsoft quickly grow the number of apps available in its Marketplace since there is a broad base of Visual Basic developers.
There appears to be some demand for more applications on WP7 devices. Last week three developers released software that lets WP7 users "sideload" applications. With ChevronWP7, users can load applications that are not approved by Microsoft in its Marketplace.