It looks like Vonage is looking to get its own iPhone application through the App Store approval process. According to Apple spokesperson Natalie Kerris, the application has been submitted but there are still some technical issues to be resolved.
Vonage would be the second high profile VoIP application on the iPhone after Skype along with many lower profile apps like Nimbuzz and iCall. Apple's placement of Google Voice into App Store purgatory has fueled a lot of industry critism and an FCC investigation.
Apple has stated time and again that it can't let VoIP applications go over AT&T (or other carrier networks). Jailbroken iPhones can circumvent this art
