Presume that we have a model named Image, which is our target for attachment_fu. Adding URL upload capability is surprisingly simple:

class Image < ActiveRecord::Base

	# Standard attachment_fu inclusion here
	has_attachment :storage => :file_system,
		:content_type => :image,
		:resize_to => "1024x1024>",
		:path_prefix => "public/images/cache/attached",
		:format => "jpg"

	# Allows the direct assignment of a URL to this image, which is the source image to save from
	def url=(v)
		self.uploaded_data = UrlUpload.new(v)
	end

	# Or, we can just pass a URL to Image#uploaded_data
	def uploaded_data=(filedata_or_url)
		if filedata_or_url.is_a? String and filedata_or_url.match /^http(s)?:\/\// then
			file = open(filedata_or_url)
			file.extend(UrlUpload)
			super(file)
		else
			super(filedata_or_url)
		end
	end
end

module UrlUpload
	def filename
		base_uri.to_s.split("/").last
	end

	def original_filename
		base_uri.to_s.split("/").last
	end
end

There you go. All you need now is Image.create(:url => "http://some.url/to/an/image.png") and when the model is saved, the image will be sucked down and saved. Easy!