Google, with their GMail service, is in effect adding content to emails. This has a pretty big impact on the ability of companies to control the information in their email communications. Once again, the Internet is acting to make it impossible to ever control information.
Steve Rubel points to an example where Apple sent him an email promotion for the new Intel-based iMacs. GMail puts "related pages" links to the right of the email itself. In this case, the related pages included consumer-generated and mainstream media news stories that Apple is discounting the older iMacs by $200. Apple might not have wanted that informattion right there in the fact of the potential customer of the newer product line. Apple didn't approve of that information being, in essence, part of the email itself.
I'm guessing that some big companies are going to have a big fight with Google over this sometime in 2006.