From PBUC item 6.5(a). "The pitcher shall be charged with a balk if he attempts a pick-off at first base and throws to the first baseman who is either in front of or behind first base and obviously not making an attempt at retiring the runner. However, there is no violation if the pitcher throws the ball directly to first base in this situation. Also note that there is no violation if te pitcher attempts a pick-off at second or third and throws to an infielder who is in front of or behind either of these bases (ie this violation is only in reference to pick-offs at first base)."
This same question is covered in the following thread:
http://www.officialforum.com/thread/14548
[Edited by DG on Jul 18th, 2004 at 06:36 PM]