This FuturePundit article describes a rat study demonstrating that a "PPAR delta agonist" prevents progression of rat Alzheimer-disease like dementia. Sounds like they're talking about Rosiglitazone (Avandia), a Thiazolidindione class drug  that's alleged to work in diabetes by stimulating PPAR Delta receptors and thus enhancing intracellular glucose uptake.
The theory is that defects in insulin uptake into neurons play a role in the development of Alzheimer's disease, and the plaque and tangles we see are secondary processes arising in glucose starved brain cells. Improve uptake is alleged to slow disease progression.
I follow this domain casually, and this "type 3 neuro-diabetes" theory of Alzheimer's seems new to me, but of course being middle-aged I've probably forgotten about earlier readings.
Interesting for sure. In the unlikely event it actually worked GlaxoSmithKline would have hit the jackpot and the US debt bomb would be defused. Anyway, if it's really promising we'll know within the year.
 All of this stuff is pleasantly new to me, I'm getting far enough removed from medical practice I actually get to read about new physiology periodically. That's fun.
PS. I've always been suspicious that exercise really helped prevent Alzheimer's disease. It would fit with this though, if exercise had some global effect on insulin response and glucose uptake ...
PPS. All members of this class of medication cause weight gain ... Isn't that a deliciously nasty choice? You can be thin and demented, or hefty and not demented ...
Update 4/21/2010: Didn't do so well in human trials!