Not content to wait until it went official, an industrious Android user carved Yahoo's forthcoming Yahoo Messenger app out of a MyTouch ROM that leaked yesterday. It can do video calling over WiFi and 3G no sweat.
Artem over at Android Police tried it out on his Evo and said it worked splendidly:
Interestingly enough, the video coming out of the 1.3MP front-facing camera was sharper than what I was seeing from the rear 8MP one. Either way, it was pretty highly compressed – Yahoo clearly went for speed and lack of lag at the expense of quality. Turning on WiFi didn't change the quality – it was still pretty washed out.