Thanks for resurrecting this post. I've been gathering some good ones I've heard around the department. To share a few:
A: Whatever, pi is basically one.
A: I've got the answer, except I'm off by a factor of 2 or so.
A: Make that 2^17.
And perhaps the best quote ever:
A: Vet school? That's just farmyard animal decomposition!
B: Don't forget: Bessel functions for cows, Legendre's for pigs.