Mutational bias suggests that replication termination occurs near the dif site, not at Ter sites: what's the Dif?

In this issue of Molecular Microbiology, Hendrickson and Lawrence analyse the sequence of bacterial genomes to map the historical traffic pattern of chromosome replication. Their surprising conclusion is that most forks terminate at the dif site rather than at the Tus/Ter sites where most investigators have concluded termination occurs most frequently. What make this analysis novel are the methods and the revisionist hypotheses for how and why forks might stop at dif.