For a while now, consumers and developers alike have been somewhat annoyed with Apple's (or AT&T's) decision to ban VoIP calls over 3G when you're on an iPhone. It means that you have to be within range of a WiFi signal, and when you're out and about, free ones are not particularly common. Today, amongst the announcement of the iPad, Apple announced that it has removed this restriction entirely and applications will now be able to make VoIP calls over 3G.
According to PR Newswire, only one application (named iCall) takes advantage of this possibility, though expect many more to follow suit very soon. As the general turnover for an App Store approval is a few days at least, it indicates that the block used to prevent VoIP calls over 3G was server-side, and not the fault of the software (which makes sense). Engadget has tested this and can confirm that it works, so it'll be interesting to see what happens now in terms of VoIP software from the likes of Google.
If you use iCall, feel free to start making 3G calls straight away, though otherwise you'll have to wait until other app developers submit some updates.