What does the electrical code say about splicing a neutral and a green ground wire?

Answer 1

The only place the neutral and ground, (white & green or bare), wires are connected should be at the service ground buss. (Somewhere you've no business being near if you ask this question.)
Because you specify a green ground, I'm assuming that you're talking about some form of appliance ground connection. ***DON'T DO THAT.***
While the neutral is grounded, at any given point in the system it may well have a potential to local ground. This could be dangerous in special locations,- just the sort where ground wires show up.
Devices like spa's etc. are grounded for a reason, and are usually associated with GFI breakers which will not operate properly if the neutral is compromised as you propose.
(AFI breakers will also be affected.)

Answer 2

I agree with the previous two answers.
However I wonder why you are asking, if for example, you are working on an older electric clothes dryer or electric range and see the neutral (grounded conductor) attached to the equipment grounding terminal of the appliance, you are seeing an exception that was allowed in older versions of the NEC (section 250-60 in the 1990 code) I believe this is no longer allowed, but you might be "grandfathered" I think four wire receptacles and cords are now required (but I don't have a current code book handy)