Has not arisen yet as the character that have died so far have stayed dead as hte players wanted to try new characters.
We haven't been in a position where taking only a hair or something like that was the best we could do, so that hasn't come up.
Some players argued that they could use any part of a body, and hence left a clipping of their hair with a trusted temple or the like. In one case, a player argued that you could use raise dead on a single hair. This prompted my own campaign's guideline of exactly how much of a body was required.
but IMC, Getting Raised from the Dead is a once in a lifetime happening and the ritual is not readily available. Maybe a handful in the world can cast it, and getting them to cast it will require at the very least a difficult quest by the other players.
Pieces removed before death are not "a part of the corpse", so that unambiguously won't work.
Haven't actually had a character die in 4e yet. Came very close a few times, but didn't get the third strike...