I was watching the all-star pregame show on TNT and I actually saw an HD commercial for Autotrader.com. Is this a new thing, or have they always been showing HD commercials?

Commercials they can show. It's the programs they have problems with :)

They've been showing some HD commercials for a while. But with all the stretched commercials they are usually too hard to spot as I skip through the commercial breaks.

depends on the format of the commercial the advertiser sends TNT.

TNT is setup so that if they receive a 4X3 'standard definition' commercial they upconvert/stretch it to 16X9 to fill the widescreen.

if they receive a letterbox standard definition commercial they upconvert and oversize it to fill the widescreen (they don't have to stretch it as it is in the correct aspect ratio)

if they receive an HD spot, they can air it in all its glory.