Someone needs to take Colorization Using Optimization and hack it into a tool for munging Flickr photostreams. In fact a general tool, or Photoshop plug-in, for pulling down Flickr photos, hacking them and reinserting them would be pretty useful.
For the intrigued, the referenced paper allows end users to do some color scribbling on a monochrome image, followed by an automated pass to colorize the image in its entirety. Seems to generate pretty good results and to also be extensible to video. Kewl!!