Skype 3G comes to the iPhone. And it's free.

This weekend Skype updated its iPhone application to let users make free calls over the AT&T 3G network. The app will be free until at least August, when Skype will begin charging a 'small monthly fee' for service.

On Saturday, Skype released an update to its iPhone app, which will allow users to make and receive calls over the AT&T 3G network. The good news: 3G calls will be free. The bad news: 3G calls won't be free forever. According to the text accompanying the update, Skype-to-Skype 3G service will be free “until at least the end of August 2010, after which there will be a small monthly fee."

(Skype newbie? Click here for a primer.)

No word on how small that fee will be, but we're going to guess that Skype is planning on offering a flat rate – maybe 10 or 15 bucks a month. For many users, of course, 3G VOIP service through an iPhone app will be a bargain, even if Skype charges $30 a month. As we've reported in the past, iPhone call quality on AT&T is notoriously spotty; Skype and other VOIP apps give iPhone owners a chance to bypass the AT&T voice network, and thus avoid the dropped calls.

That hasn't exactly made Skype – which has millions of fans worldwide – a favorite among the major carriers. Speaking to the New York Times earlier this year, Ben Verwaayen, chief executive of hardware manufacturer Alcatel-Lucent, boiled the future of the VOIP market to a simple business equation. “If everything is free, then operators will not be able to survive," Verwaayen said. "The battle is not about technology but the business model.”

In February, Skype announced it had partnered with Verizon to bring a Skype app to a range of Verizon-powered smartphones, including the BlackBerry Curve and the BlackBerry Storm. The BlackBerry Skype app works over 3G and Wi-Fi connections – and yes, it's free. Or mostly free. All VOIP apps consume data minutes, so if you're thinking of downloading Skype to your mobile phone, consider investing in an unlimited 3G plan.