Same here. Also when I try to edit. It says "no file selected" for the image section. Then uploading another image will result in showing of two images in the edit page, but still no image when previewed.

drupal 5.7
Ubercart 5.x-1.0-beta4
Image Field 5.x-2.0-rc2
Imagecache 5.x-1.3

UPDATE: installed the dev versions of image field and imagecache. Got it to work after going to site config > image cache > adding the following preset namespace:

product_list
product
thumbnail
category
cart

then you use add a new action to make it the size you want...