HI: I have that problem, my project default is in 4:3 aspect, I work in that setting, my video source is 16:9 aspect, but in premiere cs6, the final video monitor, the video automatically fit all the 16:9 aspect in 4:3, consequences, the image seems thin. This no happen in cs5 - cs3,
no find some setting relationed, so if your have some answer I'll apreciatted
VIDEO OUT - FINAL RESULTS
Right click the clip in the bin and select Modify>Interpret Footage... Make sure it's set to the correct Pixel Aspect Ratio.