According to the Power Information Network, buyers are moving away from luxury cars and SUVs towards lower-end modes of transportation (see the numbers here). High fuel prices are to blame, but does that really make sense? A so-called "top-level" luxury car is going to carry a price somewhere north of $50K. That doesn't strike me as the sort of price range where folks are real sensitive to fuel costs.
I'd like to offer up an alternative hypothesis - is it possible that people are moving away from luxury cars because "average" vehicles (such as the Acura TL) have more to offer than a luxury car? From a reliability standpoint, the majority of luxury cars find themselves trailing the popular family sedans. Luxury vehicles offer little advantage in the "standard features" category nowadays; there's a lot of safety and convenience items to be found at the $30K price point. The performance of family sedans such as the Accord V6, Maxima, and Avalon leaves nothing additional to be desired for most any four-door driver. And then there's depreciation and non-fuel-related operating costs. Obviously, though, the luxury car manufacturers would much rather place the blame on external factors such as "fuel costs" than to 'fess up to the fact that some very ordinary vehicles have caught up to them. As I said - this is just a hypothesis without much data to back it up, so take it for what it's worth.