Since Stack Overflow now has a large programmer population and a high degree of activity, it would likely be a good place for a language use index such as the TIOBE Programming Community Index. While this could be done via screen scraping the Tags page, or though the periodic database dumps, it seems that it would be a bit easier to be developed by someone with direct database access.
Additionally, a site such as Stack Overflow might be able to provide a more accurate picture of language since it reflects active issues by developers in an developer environment.
Update: One small update to this is that this wouldn't necessarily have be a real-time listing, it could also be a monthly listing like the TIOBE index which would also serve to smooth out any "bumps" in the actual use. Also, a couple responders have noted that there would be a skew towards .NET languages; however, this skewing may or may not be such a bad thing as it could also reflect the languages that are being used more in professional environments.
Update Two: Also, another thing to note as it might not be clear enough, but I'm not really referring to an industry wide standard index based upon the Stack Overflow data but rather an index that is more specific to Stack Overflow. Maybe a language/tag use trending chart is a better phrase to use?