Provided the installation is in nonmetallic conduit for its entire length, does not enter a metallic wall at the other end, and the conduits are installed in close proximity to each other, this is permitted according to 300.5 (I), Exception #2 (2005 NEC).
I still prefer to group all phases, neutral and ground into each conduit. Start going down this road, and you get others thinking there's nothing wrong with doing this, say, in parallel runs of EMT. I don't know how many times I have brought 300.20 to the attention of another sparky only to get scoffed at, especially since I look kind of young.
Funny thing, though. The smaller wires landing on the grounded bus in the back of the gear: all taped gray. In some cases, you can size a neutral according to Table 250.122 if the neutral load does not exceed the ampacity thus derived, but where's the EGC?
If there are no issues with inductive heating, I figure that separating the phases makes for a safer install. If something were to damage the insulation in one of the conduit sections, you won't be left with a phase-phase fault.
Good point winie. But if there is no fault there is no way of knowing there is a problem, and the remaining parallel runs will have to take the added load. In which case there will be some overheating.