It's not a wifi or 3g thing. It's just that this gtalk software wasn't meant for a phone just yet, so it's still not working perfectly. Therefore, the person you're calling can't see you, nor can they initiate a video chat with you.
I found a half solution to this problem. Download and flash this in recovery
http://www.sendspace.com/file/8i4ebi
Of course make a backup before you install anything.
This fix allows the other person to see me and start a vid chat w/ me. However, the orientation of the video chat is turned sideways, so one person in the conversation will just have to deal w/ seeing the video rotated. But some video is better than none at all, right?
Another thing is that this patch screwed up my gmail, so you might have to install/delete/reinstall gmail app at least once.
Lemme know if this works for you too. It works fine for dasbamf gbremix 2.1 & mr2