I'm trying to send a file to someone else via iChat, but it always fails. We're both using our Gmail accounts. Is it even possible with Gmail accounts? We also tried jabber accounts as well, but no luck. We both have Lion 10.7.2 and temporarily disabled the OS X firewall. Both our routers using NAT. By the way, video chats work (bad quality though) and iChat screen sharing works sometimes.
This link is old but may help, http://www.macobserver.com/tip/2007/02/14.1.shtml
I also noticed that it might help to just drag and drop the file you wish to send over the person you wish to send it to in the drop down chat menu and drop it there instead of on the chat box, it will then ask you if you want to send, click yes and it should work.