I think this is new to the 2.5 pre-releases, but in the Manage section of WordPress admin, it looks as though it would be pretty easy to do an accidental check all then delete. There doesn't seem to be a warning box like the one that pops up if you try to delete a post on the edit page.
I know you'd have to be pretty stupid to do this, but I'm betting with Murphy's law on this one. Someone will do it and then be complaining that their posts are deleted.
At least one confirmation of delete box should be enough to avoid most accidental deletions I think.