If you are buying new, now is the perfect time to buy a new 2011, assuming something you are looking for is still in stock.
The incentives on the new 2011's will be ending right after the first of the year and some manufacturers may not renew them for leftovers in 2012.
This, combined with the fact that dealers don't want to pay taxes on these vehicles by having them in stock in 2012 and many dealers and manufacturers have sales quotas they would like to meet make the last week of December the perfect time to buy a car @ most new car dealerships.
If you buy new, I suggest you check out the site
www.truecar.com.
If you are buying used, it doesn't really matter.
Whenever demand is low is typically the best time to get the best deal.
For example, if you are buying a RWD sportscar or a convertible in Minnesota, then December-February will be the best time to purchase.
If you are looking for a gas miser, now is also a good time to buy because gas prices are 'lower'.
As soon as gas climbs over $4 a gallon, again, the prices on fuel efficient vehicles goes back up.