The use of a 'common neutral' between _multiple_ sets of ungrounded conductors (as opposed to the ordinary use of a neutral in a multiwire branch circuit) is mentioned twice that I can quickly find in the 2002 NEC: 225.7(B) for outdoor lighting equipment and 215.4 for feeders.
In 225.7(B) the 'common neutral' must be sized fo be not less than the maximum net computed load current between the neutral and all ungrounded conductors connected to any one phase of the circuit.
In 215.4 it simply says that you can have no more than 2 or 3 three wire feeders or 2 four wire feeders sharing a neutral, and doesn't say anything about proper sizing.
The commentary in the McGraw-Hill NEC Handbook says 'A common neutral is a single neutral conductor used as the neutral for more than one set of feeder conductors....A common neutral may be used only with feeders. It may never be used with branch circuits. A single neutral of a multiwire branch circuit is not a "common neutral" '. I personally don't trust this particular Handbook, but it is what I have on hand.
I don't see anything explicitly prohibiting or allowing the use of common neutrals in article 210.
-Jon
