When I recently wrote about the Belkin Skype phone, I didn’t realize I’d be writing about Skype again so soon. But when Skype releases an app for the iPhone, it’s worth writing about again.

Skype, for the unfamiliar, is a service that lets you make phone calls through the magic of the Internet, using a technology called “Voice over IP”, or VoIP for short. If you call someone else with Skype, the call is free; there is a small per-minute charge to call someone without Skype. (Skype itself is a free application.)

One important point to notice is that you can only use the Skype app to make calls when you’re in range of a wi-fi network; Skype does not use cellular networks, so you can’t use it in your car or other places that are out of range of wi-fi.

iPod Touch owners can also use the Skype app, but they will need to use headphones with an included microphone to make or receive calls, since the Touch doesn’t normally have phone capabilities.

CNET is reporting that the Skype iPhone application will be available Tuesday, March 31. I’m assuming the Skype app is free, although news reports don’t specifically state that information.