I too have this problem. I shot in 720p24 on the HVX, edit on a DVCPROHD 720p60 timeline (Pixel Aspect: HD 960×720). The aspect looks good in the viewer, but when I export in Quicktime or export a single image (PNG or jpg) it gets squished. BTW, this is FCP 5.1.2.
Viewer in FCP:
-> https://dev-null.com/fcp.jpg
Exported image:
-> https://dev-null.com/fcp_export.png
Any thoughts?
Jub