It's been a long time coming, but the last hurdle is gone -- Apple has lifted the restrictions against 3G VoiP calls from the iPhone Software Development Kit.
The implications of this are pretty huge. Not only does this mean that VoIP apps like Skype [iTunes Link] can now be easily updated to allow VoIP access over 3G on the iPhone, it also means that the 3G-enabled versions of the new iPad should be able to take full advantage of VoIP over 3G as well. The iPad does come with a microphone and a speaker, so while you won't be able to hold the device up to your ear like a phone (although it would be pretty funny to watch you try), calls over Skype or another VoIP program should theoretically be just as clear on the iPad as they are on the iPhone's tiny speaker. This has the potential to really shake up the wireless industry, especially in the U.S. -- with pay-as-you-go 3G on the iPad costing no more than $30 a month, the iPad suddenly seems like a very attractive alternative to the iPhone with its far more expensive (and contractually obligated) carrier plans.
iCall [iTunes Link] has already been updated with VoIP over 3G enabled, and with all restrictions now lifted, it seems like it's only a matter of time before Skype and other VoIP apps follow.
UPDATE: Fring announces the availability of their iPhone app, now with 3G calling and video.