[drjones]

Basic question:  What is the difference between a mash that starts at 148 and rises to 158 over an hour, vs. one that starts at 158 and drops to 148 over the same time period, assuming a comparable rate of temp change over time?  The risk of the high temp dough-in seems to be the increased potential to denature beta-amylase, leaving only alpha-amylase.  At what point is this really going to happen?  158F, 162F, or higher?

[a10t2]

You'll get a more fermentable wort out of the conventional mash schedule.

http://seanterrill.com/2011/10/15/reverse-mashing-2/

[drjones]

Thanks, Sean - that's a great link.  Nice work.
I'm still curious about the denaturing rate of beta-amylase at the upper end of its range.  It seems like enough survives a 158F dough-in if the reverse mash cools from there.  Clearly, the resulting conversion will be biased toward less fermentable sugars, but it seems there's plenty fermentable goodness left when all is said and done. Sounds like one should expect a couple of points higher FG, but not, say, 10 points higher.  I suppose if one were to mash-in at about 160F the results would be more exaggerated - though Lagunitas is making some pretty good beer in this temperature range, if what I hear is true. 
Of course, this all comes form the desire to be lazy, and just let the mash do its thing with less tinkering.