It takes a while for changes like this to show up. Remember the gas price hike has been very recently and ppl will wait a bit to see if it stays high (it will, IMO), and if it does, then you will see a shift to smaller, more efficient vehicles by some percentage of the ppl; others will just go on paying the price (and likely complaining about it).
As an automaker, it might be a good idea to put any plans for additional large trucks (personal use trucks), SUVs and Crossovers on "hold" for awhile. Jeep is committed on the new Commander, Ford has a new, larger Expidition coming and GM has the '07MY large SUVs already done. Can't stop those now; but sales may be dismal or at least well below expectations.
Given it's NA with areas of large, wide-open spaces, there will always be a market for some larger vehicles, although diesel and hybrids may become more common.
We'll see.