When I deactivated Custom Field Template (which is an awesome plugin!), I was able to insert an image just fine.
win.send_to_custom_field throws the error (doesn't exist?).
I'm not sure if this is related to the issues discussed on this thread: http://wordpress.org/support/topic/317991/page/2
In any case, for this particular site, we're manually setting up the one field we were using the Custom Field Template plugin for, but I thought the info might helpful in case other folks are running into this odd interaction with WordPress 3.0, custom post types & inserting images, and the Custom Field Template plugin.