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 Wi-Fi 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.