have you made sure the photoshop file is using the correct PAR (pixel aspect ratio)? you can change it at the bottom of the window, in the advanced tab, when starting a new file, or if you already have the new file created you can change it under Image > Pixel Aspect Ratio. Both standard definition 4x3 and 16x9 NTSC use 720 x 480 as the aspect ratio. the only difference is that 4x3 uses a PAR of 0.9 while 16x9 uses 1.2.
Not sure if that'll help since I've not used DVD Studio Pro before, but I create DVD's for work and private use all the time with either DVD Maestro or Adobe Encore and never have a problem as long as the PAR is correct.