I've played around with something like this before, and indeed I think it's pretty cool, and can be done with any image. The one big downside is that the "size" of the image, in terms of downloading it, is much larger than the image you're replacing. With a gzip enabled server, that might not be so bad however.

On the other hand, although this is a cool hack, I really hope it doesn't catch on. I usually browse the web with images off so I can skip the annoying advertisements that represent probably 80% of web images nowadays. With this technique, there's no good way to turn it off except to disabled displaying divs. And that would probably be a nightmare. I could see unscrupulous advertisers using this technique to get around image blocking and such.

PS. For some fun playing around with this for any random image, download bmp2html. You can modify the source to spit out colored divs, like your program does, instead of colored ASCII characters.