Acceptable ground to neutral voltage?
The best I could do was to dig up a fine print note (FPN) in the National Electrical Code (NEC). In theory, there is no limit (as stated in other answers). In practice, the limit is when things stop working. In this FPN, the limit is 3-5%. In reality, the limit is around 1-2%.
NEC 2008

210.19 Conductors — Minimum Ampacity and Size.
(A) Branch Circuits Not More Than 600 Volts.
(1) General.
FPN No. 4: Conductors for branch circuits as defined in Article 100, sized to prevent a voltage drop exceeding 3 percent at the
farthest outlet of power, heating, and lighting loads, or combinations
of such loads, and where the maximum total voltage drop on both
feeders and branch circuits to the farthest outlet does not exceed 5
percent, provide reasonable efficiency of operation. See FPN No. 2 of
215.2(A)(3) for voltage drop on feeder conductors.

This Fine Print Note (FPN) says that to provide &quot;reasonable efficiency of operation&quot;, the voltage drop of the branch circuit to the furthest point should not exceed 3%. And that the total voltage drop, including the feeders should not exceed 5%.
120V * 3% = 3.6V
120V * 5% = 6V
Given this information.  You should start to examine your wiring, if you measure anything near 3V neutral to ground.
Note: Fine print notes are informational only and are not enforceable as requirements of the National Electrical Code.