I've heard rumors of terrain adjustments to control times in the Western U.S., but I've not seen them myself there or anywhere else. The 2009 GRR didn't have any terrain adjustment that I was aware of. Like the other 1200s I've done, riders had 40 hours for the first 600K and 50 hours to finish from there; I don't recall any of the intermediate control times being adjusted for terrain.
Note that some RBAs are likely more generous with time credits than others, but that the "best practices" approach to riding brevets would be to ride assuming that you're not going to get any "extra credit" at all, for anything. Not only do you need to plan for mechanicals and other totally foreseeable circumstances that might delay your arrival to a control, but also make sure you have time to deal with detours. Though some riders seem to think the RBAs should drive the route the day before the ride, this is rarely the practice. Road construction is common in the summer in the U.S. and it's often the case that a bridge that was there last week is gone on the day of the brevet or that a road is no longer paved because the work crew stripped it right down to the clay underlayment. Having a detour -- even a long one -- in the midst of your ride is foreseeable and I don't think most RBAs would give you any time credit if the detour caused you to be over time. For those who regularly press up against control closing times, keep this in mind.
My memory of PBP in 2007 was that they did not enforce intermediate control closing times after Loudeac out-bound, within some reason, but that you had to finish the event within time overall. I recall all sorts of rumors circulating in the midst of the ride about 1- or 2-hour time credits being permitted (making the ride 91 or 92 hours rather than 90), and recall lots of moaning at the end of the ride because some felt that they'd been misled about how long they had to finish. That sucks, but I think the take-away for future rides and riders is that unless you have an entry in your brevet card giving you extra time then you ought to ride like you've got 90 hours. (Personally, I think the idea of permitting extra time because it was rainy is pretty silly; this is randonneuring ... It rains!)