According to a post on 0xDECAFBAD, Safari records a bit of extra metadata onto every file that you download:

$ mdls QS.2AD4.dmg 
QS.2AD4.dmg -------------
kMDItemAttributeChangeDate     = 2005-05-06 12:40:37 -0700
kMDItemWhereFroms              = (

Notice the kMDItemWhereFroms entry–that’s Safari tagging the file to remind you where it came from. I didn’t think that was possible in Tiger–Safari just downloaded the file, it doesn’t own it, and it doesn’t provide a Spotlight indexing driver for the file. Somehow, it still managed to add extra metadata of its own onto the file; I though we were going to have to wait for 10.5 before we could do this.

Unfortunately, the extra tags are fragile. I made two copies (one using the Finder, one using cp), and neither copy has a kMDItemWhereFroms field. That’s what you’d expect if Safari cheated and stuck an extra field into Spotlight’s DB, but it’s not how a “real” metadata system would behave. Hopefully 10.5 will allow metadata to belong to the file itself, not just the Spotlight index, so that copying it will preserve the extra metadata.