And I'm one of the crowd who is wondering how long it'll last. It seems designed to be denied / ejected.
The Line2 app is a carbon copy, a visual clone, of the iPhone’s own phone software.
Line2 was first submitted to the App Store in August 2009, which was really bad timing because it was in the middle of the Apple/Google Voice storm. A month later it was approved (after the founder pestered Apple a lot). And I'm pretty sure the version Pogue is reviewing was actually released last month (http://techcrunch.com/2010/02/18/line2-google-voice/).
In other words, none of this is news to Apple, though it's clearly hypocritical on their part. Also, no way Apple would pull this after a Pogue review — he'd probably write about it, and he's proven before with his 'Take Back The Beep' campaign that he can incite quite an uproar.
Edit: Never mind, I should have finished reading the article before posting. Line2 routes calls through the cellular network when the app isn't running.
Input and output
* Dock connector port
* 3.5-mm stereo headphone jack
* Built-in speaker
* Microphone
* Micro SIM card tray (Wi-Fi + 3G model only)
It's likely the headphone jack is even iPhone-compatible (accepts headsets with inline microphones), though the wording above isn't clear.