As noted in an earlier hint, Tiger has added the "Where From:" field to downloaded files. Unfortunately there is no easy way to open these unselectable URLs in Safari.
This script solves that problem by popping up a window showing all the available URLs for the currently selected file. Double-clicking a URL will load it in Safari.
try
tell application "Finder"
set theFile to selection as alias
set theFile to POSIX path of theFile
end tell
set AppleScript's text item delimiters to """
set theInfo to (do shell script "mdls -name kMDItemWhereFroms " & quoted form of theFile)
set url1 to (text item 2 of theInfo) as text
set url2 to (text item 4 of theInfo) as text
set theURLs to {url1, url2}
set theURL to (choose from list theURLs) as string
if theURL is not "false" then
tell application "Safari"
make new document at end of documents
set URL of document 1 to theURL
end tell
end if
set AppleScript's text item delimiters to ""
on error
display dialog "Sorry, no "Where From" URLs were found."
end try
[robg adds: I tried this, and it worked great. I saved the script as an Application, with Run Only and no startup screen. I then put the script in the toolbar, selected a download, and clicked the toolbar icon. Presto, a floating URL box shows up listing the two URLs embedded in the chosen file's WhereFrom field.]

