Very similar to my other topic on Framerates, MPEGs and VOBs but created as a separate topic as my workflow and the symptoms are different (although they have the same solution).
I created an 80 picture slideshow in Corel VideoStudio Pro X4 Ultimate (Corel really should fire the guy who dreamt that title up). It is a 4:3 ratio and it uses Pan & Zooms in just about every shot. There is a transition effect between every picture. I exported the show to a widescreen MPEG and when viewed in VLC or other software players it correctly shows in pillar-boxed format. Note that, unlike Adobe PRE, Corel VSPX4 project settings are not so critical as they can be changed dynamically.
I load the MPEG into my PAL Widescreen DVDAS5 project and yuk. The 16:9 ratio of the MPEG (verified with GSpot) is stretched. After my success with the framerate issue (discussed in another topic) I used the VSPX4 burning engine to output a 16:9 DVD to folder, dragged the VOB into DVDAS5 and its perfect.
Now it seems to me that with just one event the culprit could have been with the PRE10 or VSPX4 output. But for the output from two different video processing products to be resolvable through using VOBs rather than MPEGs does suggest to me that the problem is with DVDAS5.
Just an observation, no problem as it all worked out fine. And, as a tip, if you have MPEG problems with DVDAS5 try using a VOB instead.