420, are you sure that the thumb isn't getting created? When I was doing batch uploads I encountered the scenario where occassionally on the batch-upload edit screen I would get either a red-x or the generic file image but after I hit 'process' the thumb would get created. Was happening with one of the betas.
But, either way... if the only problem is that the thumbnail is missing and the image is definitely getting saved to the database then you can just rebuild thumbnails from the ACP.
Edit: Also, try a smaller batch size and go from there. For example, if you choose to process 5 at a time does the problem still occur?