It's one of those obvious things that makes you wonder why someone else hasn't done it.
Paid Content (whom I'm helping put together put a conference and have contributed to) today launched a Media Index of public companies in the media industry. Much different and with fewer companies than the Dow Jones Media Index often cited in The New York Time. Times' considers "media" in the much more global sense -- everything from TV and Radio to newspaper distribution and even some e-commerce. 9I see the index on the Times' page, but there's no chart on Marketwatch when I click through.) Paid Content's parent, ContentNext, has a much more honed list of digital media companies and those that serve them, such as Adobe (makers of Flash and other nifty software and applications).
Indices are useful as an indicator, but I discovered in biz school that it can actually be difficult as an individual investor to buy according to an industry index, because a fund with an appropriately weighted average of shares in each company may not have been created and made available.