I can't remember the exact case but the U.S. has done this before, IIRC, to Canada. Something about a state extraditing someone for trial in a state court (with a firm agreement to leave off the death penalty) and then the Federal prosecutor swoops in out of nowhere to bring Federal charges that do carry the death penalty.

Edit: Take the above with a grain of salt though, I've spent some time searching for this case (which should be quite famous) and have come up with nothing. The closest was the Söring case where the U.K. has to obtain assurances from both Virginia and the U.S. that the death penalty would not be applied... and it wasn't.