I did not dare to believe it but today Vonage launched VOIP clients for the Apple iPhone and also for BlackBerry phones.
Vonage Mobile for the iPhone is free to download and once you've installed it you are ready to start calling. Vonage gives you a $1 (kind of symbolic, but whatever) credit to make your first international call. Vonage for the iPhone works on Wi-Fi and over 3G.
I am installing the Vonage Mobile for the iPhone right now and will be back later with an review. I have been using Vonage for years and if it works well on the iPhone it would be really awesome.
More details on the Vonage site. Update: My Vonage App Review, if you can call it that is up now.