In the "old days", there was a lot of RS-232 and similar stuff running around. RS-232 connections are direct electrical connections between the equipment, using the equipment ground as a voltage reference. A small difference in the ground voltage can kill equipment like this, and it doesn't take much noise in the grounding system to cause data errors.
More recent stuff, like Ethernet, uses differential, transformer-isolated connections. These transformers are rated to withstand a 2,000 volt potential difference between the communication line and the equipment. Because the connection is differential, it has something like a 60-90 dB rejection of common-mode voltage. It's really hard for a grounding problem to cause a problem in the equipment. And even if really severe noise on the grounds causes occasional data errors, most modern communication protocols have error detection and recovery mechanisms built in, so errors normally just result in a minor degradation in throughput.
Interesting historical note: When Xerox installed their first prototype Ethernet network, they found that there was something like a 90V difference in ground potential from one end of the building to the other! That affected our thinking about isolation, so when we designed the 10 megabit Ethernet standard, we made sure that everything was transformer isolated.
[This message has been edited by SolarPowered (edited 11-12-2004).]
