FogBogz calls this Evidence-Based Scheduling. They take past estimates along with data on the actual time taken for past tasks and use that to predict the time required for future tasks. Caveat: I have never used it and therefore cannot vouch for how well it works. The technique appears sound.

More info http://www.fogcreek.com/fogbugz/evidence-based-scheduling/ and http://fogbugz.stackexchange.com/questions/4396/how-does-evi...