I've hear it as, "the first 90% of the task takes the first 90% of the time, and the last 10% of the task takes the other 90% of the time." :)

The critical thing many people miss about this rule is that it applies recursively. So of the 20 % that takes 80 % of the time, in turn 20 % takes 80 % of that time.

Go further and you get a 50-1 rule: 1 % of the work takes 50 % of the time. (Or, equivalently, 50 % of the time gets you 99 % of the way there.)

This is a strong argument for clever scope reduction.

Edit: To be clear, this is the Pareto distribution with α=1.16 -- and I don't think this applies to software implementation. The tail isn't quite that fat, in the data I've had access to. But the general idea still applies, except with less aggressive percentages!