I don't have my code book handy but am reading from a print out of major changes in the 2011 code. It says, "Will not allow a neutral conductor to be used for more than one branch circuit or feeder". Does this mean no more shared neutrals on 14/3 or 12/3 ? thanks..
The rest of that sentence answers your question....
Neutral conductors shall not be used for more than one branch circuit, for more than one multiwire branch circuit, or for more than one set of ungrounded feeder conductors unless specifically permitted elseware within this Code.
from the '11 NEC Handbook; NOT commentary, Code text.
The short answer is a multiwire circuit is ONE circuit. That can be 2 ungrounded conductors in 120/240 or 3 ungrounded conductors in 208 wye. There are requirements about how and where these conductors have to be grouped and how overcurrent devices are coordinated (getting tougher each cycle it seems) but it is still one circuit.
This discussion reminds me of one of my 'creative' moments. I was under the impression that the new code language made my otherwise legal solution forbidden.
It was a small house, a complete rewire. Construction was solid masonry. Fortunately, there was an attic.
Even so, the large EMT (not sure of the size)I ran out the side of the outside panel, up the wall, and into the attic was pretty tight. So, to reduce my wire fill, I placed a can in the attic and mounted some terminal strips. I landed my grounds on one strip, and my neutrals on another. I did run a #10 ground wire (redundant to the conduit) and, I believe, a #6 neutral (the largest circuit was 20-amp, and circuits were split between legs) from the can to the panel. Thus, that pipe had many 'hot' wires, but only one ground and one neutral.
It was an exceptional situation, and I have not used this method again. Under the current code, I am clearly not allowed to do this.