I found a statement on FEMA's site that said, "Areas with a 1% annual chance of flooding and a 26% chance of flooding over the life of a 30-year mortgage." The statement seemed rather odd; a knee-jerk reaction would be that the figure should be "30%". Then I saw that they took the approach of: 99% chance of staying dry in the first year and 99% chance in second year and... = 0.9930 = 74% chance of staying dry over 30 years, so 26% chance of flooding. Still, even that doesn't quite seem right, since the same logic would yield a 63% chance of flooding from a 100-year flood over a century. OK, 100% seems wrong, too, but 63% just feels too low.
Of course, now that we're getting 100-year floods every few years, all of that seems moot, anyway. ;-)