Derivative of round (or nearest integer) function

From: IdoNotWantAnySpam (matlabber_at_yahoo.com)
Date: 12/22/04 

Date: 21 Dec 2004 20:11:14 -0800

How can one calculate the derivative of the round function? (e.g.
Round(2.2)=2, Round(2.6) = 3)?

Thanks