Anyone else seeing this? I'm using both superslider-media-pop and faster-image-insert, so those may be a contributing factor but 2.9.2 showed the thumbnail in the post edit screen, as expected. The thumbnail version does show in the post itself, thankfully.
Update:
In case anyone else runs into this, it's superslider-media-pop. Disabled both of the above and WP3 displayed the correct thumb size in the post edit screen. Then enabled one at a time and SSMP is the culprit. I'll report it, then see if I can figure a hack.