What particular Cisco device do you have in mind? For devices that have multiple power suplies with a separate power cord for each (i.e., CAT6500, CSS11506, PIX535, etc.), there's no reason why you can't supply them from separate phases, as long as the voltage/current requirement of each power supply is met. It's DC on the inside, so the phase doesn't matter. On the other hand, if you're looking at smaller devices that have only a single power supply, you can only connect them to a single phase.
The 7600-series routers and the 6500-series switches use the same physical components. The hardware & installation documentation for both of these product lines fails to mention AC phase anywhere. I think it just never occurred to the writers to say anything. Given that they DO supply warnings for other potential electrical problems (i.e., they stress that all supply circuits must be fully grounded), one should be able to deduce that if phase mattered, they would have said so. As I mentioned before, there really would be no logical reason for a device that runs on DC internally to have any AC-side interconnections between its power supplies, anyway.
My team installed about twenty 6509 & 6513 switches this summer, and we never even considered phase. We mixed & matched them in the lab without consequence, and when doing final installs, just got power from the datacenter managers, from wherever they had it available. One handy thing to bear in mind is that some of the newer 6500/7600 modular supplies are rated for a wide range of input voltages. If you supply them at 208v (phase-to-phase) instead of 125v (phase-to-neutral), they can supply more amps DC output, and will let you run more slots of components in fully-redundant mode. Use "show power" command to see details.