While doing my first ASP.NET 2.0 "publish" tonight I was presented with the following dialog box:

Assuming that apps delete as few items as possible, I assumed that "existing files" meant files that matched those that were being copied. I was wrong. It does indeed delete everything.
I've salvaged most of the site from local copies, but most of the images that I've posted to the blog (that aren't app-related) were lost. Nothing drastic, but it is kind of funny that the one thing I've never backed up was lost.
Moral of the story: Don't be a dope.