I'm amazed by the number of people I talk to that are mixing flash with the bracketing shots they are shooting for Exposure Fusion (Blending in Photomatix) and HDR. Plus I've gotten a number of questions asking how to to shoot brackets with flash.
I've talked to people recently that seemed offended when I pointed out that adding a single manual flash to their bracket shooting could improve the quality of their results. We've talked about this several times on the blog since the summer of 2010 and in the flickr discussion group. I think there are some folks that missed out on that discussion. Iran Watson uses a variant of this technique and did a post about the way he uses it in April of 2011.
The first question is why? What's the point of using flash when you are shooting brackets? You thought the reason you shot bracketed exposures was so you didn't have to use flash, right. Well the fact is that when you use HDR or EF for interiors you tend to get low contrast results (more so with HDR than EF). That is, the blacks are not as black as you'd like and the whites are not as bright and crisp as you'd like. The term that comes to mind is muddy colors and dirty whites. You also have issues with white balance. It turns out that if you add a kiss of fill light from a single flash makes these problems much better. If you add a little fill flash to your brackets you don't have to spend as much time and effort in postprocessing to get rid of the muddy colors and dirty whites.
Also, adding a touch of fill flash to bracketed shots can be done without having to "climb to the top of the learning curve" for full blown multi-off camera flash technique. There is a discussion in the PFRE HDR & Blending discussion group on this subject. I've been doing some experimentation with this technique so here's my summary of how to bracket with flash:
To summarize: this technique adds the same constant level of fill flash with a single flash, to each of the three brackets you shot. This fill flash is improving the quality of the light so that the whites are whiter and the blacks are blacker. This same technique works similarly with either brackets used for HDR processing or brackets processed as Exposure Fusion (EF).
For those that are already shooting brackets with flash I'm sure you'll be able to add refinements or variations to this technique.