It's totally off the mark in terms of absolute numbers, especially when you take into account the huge variety of uses for a machine... my machine, for instance, isn't really bottlenecked by a 52mbps network connection (not to mention 4mbps internet), but I'd still like more than 3ghz dual core. A whitebox in a Google render farm probably WOULD be noticebly slower with 100 megabit compared to 1 gigabit on a MapReduce.

The thing is, though, the 'law' was created in the 60s, and it's stayed mostly order of magnitude correct since... so that's quite impressive in itself.