I had this same problem too... I read somewhere in the forum that it could also be caused by a specific version of imagecache/imagefield combination...

Anyway, the way I solved it was to move to ImageCache 5.x-2.0-rc2, ImageField 5.x-2.0-rc8, Image 5.x-1.8, etc.

I also recreated the products in ubercart, as the ones created with the older versions of the modules did not show the images.

Once I did this, it worked...