Neil Martinsen-Burrell just announced MCREPOGEN, a tool to generate random version control histories for Bazaar or anything that can read the fastimport format.
It uses a Markov Chain model where the states are directory trees and various changes to the tree have associated probabilities. The intent is that by giving complete control over the characteristics of the history, performance testing of different aspects of VCS can be improved.
Bazaar developers' blog 