I just came across Bishibosh's remains, which dropped Wirt's Original Leg. For those familiar with Diablo 2, you could combine a Tome of Town Portal with Wirt's Leg to get to the Secret Cow Level. In Diablo 3, does Wirt's Original Leg serve a similar purpose?
Since the secret level and the method of reaching it has been revealed (the recipe requires Wirt's Bell), it seems like Wirt's Original Leg is just an in-joke.
Since it's not an ingredient of the Staff of Herding recipe, and the internet is still silent about its use two days after release (this is an incredibly long time for a Blizzard game), I think it's safe to say at this point that it's just an easter egg, or more likely a red herring, referencing the leg in Diablo 2.
Another fact pointing to its being as an easter egg could be its naming: Wirt's original leg (the one he got before he lost his old one) as opposed to Wirt's leg.
Considering the leg is skeletal its probably his "actual" "Real leg" before he was amputated by Pepin the Healer back in Diablo 1. After its decayed and the flesh rotted away.