Plus, this patch comes with the nice benefit that "stuck" tx can be easily fixed. Though, I do agree that a non-deterministic "undo" button is a bad idea. It would be unreliable in many ways, and would also give a false sense of reality to users. Instead, I'd like to see a button/dialog that says "This transaction has been waiting more than one hour without being accepted. Would you like to increase the fee to try to speed up its acceptance?"
Yeah, the user-interface aspects will be tricky to communicate. For instance
if an undo button is implemented, it probably should actually be called something like "Attempt to cancel", and at least initially be hidden off in the RPC interface anyway.
Increasing the fee isn't such a big problem, although a dialog box that points out you have the option after some reasonable delay is a good idea. It should probably be called "Offer a higher fee" so users aren't as surprised if the lower fee version goes through anyway.