If any doubt lingered that voice is now just another feature on cellphones, the latest version of Skype for the iPhone should dispel it. Skype can now receive calls when the app is running invisibly in the background, which was a major omission in previous versions released before Apple added multitasking to its OS. Combined with the May addition of support for Skype calls over AT&T’s 3G data network — and Skype’s simultaneous decision to keep 3G calls free indefinitely — the app is now a workable replacement for the iPhone’s normal calling feature, especially for those people who have Skype accounts which provide real telephone numbers. Video calling is not included — “yet,” we hope, since the iPhone 4 is the first model to have a front-facing camera. But that feature may have a hard time passing Apple muster since it duplicates a new core functionality, FaceTime. Still, this is what we’ve been talking about when we spoke of mobile VoIP nirvana, and it puts the iPhone in the forefront of Skype handsets (in the United States, anyway). Skype mobile is available on a handful of Android and Blackberry phones with Verizon