Although Cat 5 and the recently ratified Cat 5e cable standards suffer interferrence from power cables, there is much more to it. Power and data can co-exist in near proximity, especially for short runs. If we think of plastic dado trunking we have two or three compartments, keeping power and data separate, but it is not earthed. This means in normal installation practice we ALL run power and data together within millimeters of each other. Also if you have cables running in ceiling for example, how close to flourecent lighting are they, power cables? machinery?
Cat5 is capable of 10 or 100mbps and now, Cat 5e is capable of 155mbps. The larger the bandwidth the more succeptible the cable is to interferrence.
I'll not admit this to one client in the South of England but I once ran a Cat5 cable WITH power in a plastic trunking that was, frankly, too small. For 15 metres! The PC is happy talking 10mbps. There are no traffic issues on that network. I'll be quietly interested to see what happens when they go 100mbps!
I am personally a bit sceptical of this rule. I use it as a guide. So long as the cable types are not tight together, or weaving together, or for more than a few feet. And so long as it passes my Penta-Scanner certification testing, I sleep at night! I have never dared repeat that 15 metres! I would suggest that if you are encurring no issues, just remember the rule and sleep at night.