I tested the new bulk action media management the last days to delete a massive amount of images (I uploaded them once for another test).
Well, turns out that even thoroughly checked, some pictures were deleted that were not intended to be deleted. Ironically I was deleting them because I was not able to export my blog any longer because of them.
I now need to find out which images are lost and how I might be able to recover them (looks like the trash feature we know for posts does not exists for media library items).
The good new is, that the Mass Deletion Problems with WordPress will be solved with 3.1. I could delete in 500 images at once quite well (doing about 10k files over the last two days).