Skype is set to launch a mobile application for the iPhone, a long-awaited entry to Apple's App Store, and one for BlackBerry devices in May, Skype officials have confirmed.

The iPhone application will be announced tomorrow at the CTIA Wireless 2009 trade show in Las Vegas, said Peter Parkes, who writes Skype's corporate blog.

Skype, which is owned by eBay, has been a noticeable latecomer to the App Store, which features several other VoIP applications.

One of those, Fring, uses Skype's API. People can make free Skype-user-to-Skype-user calls, or use SkypeOut, a paid service where people can call landlines and mobiles via Skype.

The iPhone Skype application can only be used over Wi-Fi and not over the cellular networks, a condition demanded by operators so as to not steal voice revenue. A spokeswoman would not reveal further details, saying the mobile application will have the "core capabilities" of the desktop application.

Skype does not yet have a native application for BlackBerry devices, although third-party developers again have used Skype's API. One of those services is IM+ for Skype, which enables chat, free calls to Skype users and SkypeOut.

Skype's application for the Windows Mobile OS has been available for several years.