For those of you catching up, you can read the Mash Thickness, Pt.1 here. That should bring you up to date right quick.
Consider the following from a BYO article by Tom Flores, brewmaster of Brewer's Alley:
"High mash temperatures favor a less fermentable wort because alpha-amylase is a lot more stable than beta-amylase is at higher temperatures. This means that there will be less production of maltose as the activity of beta-amylase diminishes. It is hard to say that beta-amylase activity will be expected to drop off at a particular temperature, because the thickness will determine what temperature activates maximum beta-amylase activity. Thicker mashes tend to retain more beta-amylase activity at high mash temperatures than do thin mashes. This is because beta-amylase is more stable when joined with its substrate than when it is not.
Because beta-amylase encounters substrate less frequently in a thin mash, there is more opportunity for it to be destabilized and inactivated."
If beta-amylase has more of an opportunity to be destabilized and inactivated in a thin mash, why does Noonan write that a thin mash tends to increase maltose production? The two seem to conflict. Is this a function of the fact that the author included the caveat "at high temperatures?" My guess is that this is the case. The quoted optimal range for Beta-amylase activity is substantially lower than that of Alpha-Amylase. Palmer notes that the range for Beta- is between 131-150F, while Alpha- is between 154-162F. If one were doing a mash outside of the optimal range of Beta-amylase, it would stand to reason that a thick mash would be conducted. Why? Just as Flores notes, it tends to destabilize when in further proximity from its substrate. It's already working overtime since it's out of its optimal temperature range. A thinner mash will just increase the work it has to do by making it "seek out" starches to break down.
This would also explain why Noonan writes that "a thick mash...induces the greatest overall extraction." Yes, it may end up being a more dextrinous wort due to the high mash temperatures, but the total end product will be of a greater extract as both Alpha- and Beta-Amylase will be working in concert with one another.
I suppose this is just one more way in which brewing is a balancing act. Mash time, temperature, thickness, pH, etc. are not isolated phenomena. They all work in conjuction and, so it may seem to a new brewer, can even work against one another in some situations. As always, these things will also depend upon the type of system you use as well. Perhaps this is why I'm having a difficult time finding hard and fast rules. That being said, there is still much research to be done.
I still haven't quite mastered the Technical Tuesday portion of this blog, though I hope it's improving.
 Flores, Tom Managing Mash Thickness (accessed 8-8-07)
 Palmer, John How to Brew (accessed 8-8-07)
 Noonan, Gregory New Brewing Lager Beer (Brewers Publications, 1996) p. 140-1