A friend of mine has already made a complete implementation of this using the code from the 3.0 trunk. Sorry, I can't remember what revision exactly at the moment.
I am pretty sure he was trying to get it into 3.0 before the code freeze but missed it, now I'm not sure what he's doing with it. He might just be sitting on it but I can find out.
His implementation was EXACTLY what I was talking about, the link name he used is simply "Detach", which also works in conjunction with "Attach". He made this functionality work across the board, in all Media Library pages, and the post attachment popup. All the listing tables show a "Detach" link where appropriate, so think where it says "Attach" on the "Unattached" listing, this would be replaced by the latter. He even has it on the attachment detail settings as a checkbox, of course this checkbox only appears for things that actually have a valid post_parent.
For the media library, he has implemented this through ajax. On the edit screen popup, it is a simple page refresh.
Along with this patch, he has also tackled the "Attach to" popup in the media library to actually list valid registered post types dynamically rather than using the "post" and "page" types statically. He also allowed the results of all entries of the selected post_type when nothing is within the search field rather than nothing.
Anyway, I would be happy to see what is going on with this, and information from someone in the core team about what I can tell him. Like if there is any chance this patch could make it in sometime
If anyone would like some sort of a screenshot, demo, or just more information in general let me know and I will relay it to him, and see if he can just try and it in there himself.
Don't mean to be the "go between", but hey, sometimes you have to ;)