This AppleScript (for use in Keyboard Maestro) will copy the file selected in the Finder to your public dropbox folder and copy the public link to the file to your clipboard. Don't forget to change The XXXXXX to your public "ID".

Move-to-Dropbox-public-folder.applescript

-- Add file to Dropbox Script
-- Andrew Burgess, modified a bit from http://www.bencardy.co.uk/blog/dropbox-applescript/

-- You'll have to change the first part of your Dropbox URL, in the filePath variable
-- Also, make sure the myPath variable points in the right direction

on splitText(delimiter, someText)
	set prevTIDs to AppleScript's text item delimiters
	set AppleScript's text item delimiters to delimiter
	set output to text items of someText
	set AppleScript's text item delimiters to prevTIDs
	return output
end splitText

set myPath to (path to home folder) as string
-- First thing: if your Dropbox isn't in ~/Documents/Dropbox, change this:
set myPath to myPath & "Documents:Dropbox:Public:"
tell application "Finder"
	set theFile to the selection as alias
	duplicate theFile to folder myPath
end tell
set fileName to item (count splitText(":", theFile as text)) of splitText(":", theFile as text)
-- Second, you need to change the first string below to your Dropbox URL,
-- it should be the same for all your public files.
set filePath to "http://dl.dropbox.com/u/XXXXXX/" & fileName
tell application "Finder" to set the clipboard to filePath as text