Skype video chats on an iPhone are now a reality--more or less.
Here's the 'more' part: You can use the free Skype iPhone app to place and receive free video calls on an iPhone 4, iPhone 3GS, and iPod touch 4G. And you can receive video calls on an iPad and an iPod touch 3G. Given that both devices lack a camera, you can't place a video call from them.
Interestingly enough, you can video chat with Skype and an iPhone over AT&T's 3G network--something Apple doesn't let you do (yet) with FaceTime. Skype video calls can also be conducted over Wi-Fi networks, and that's probably the best bet for quality.
And now, the 'less' part: I can't imagine that a video chat on an iPhone 3GS would be worth the trouble. Because that phone lacks a front-facing camera, you'd have to stare at the phone's back to project your image--which means, of course, you can't see the person you're Skype-ing with.
At any rate, I just did a quick test, using Skype on my iPhone 4 to call another Skype account on an iMac. It worked really well.