It appears Google may be starting to index non-HTML content such as JavaScript and CSS. On the blog titled, “things of sorts,” suspects this is a move by Google to go after Black Hat SEOs. The blog provides a reference to the header requests from Google against his CSS and JS files.
I don’t think Google is using this data to pinpoint blackhat SEO behaviors; however, I do think they might be making a thumbnail service or similar rendering of websites for users to view as they use their search engine. Just a theory though, I wish I had inside knowledge. It may also just be a random quirk of the GoogleBot, it may have been linked by another site and it may have caused it to index that one file.