I'd post a link but it was originally hosted on blip.tv and no longer exists online.

It's by-far the most influental talk about marketing I've come across covering the 'New Coke' branding kerfuffle as well as a lot of other great examples (ex Southern Bell's rebrand to AT&T).

The message, a brand is essentially an immutable const identifier in the minds of the users. Once people establish a firm perception about a brand, it sticks for life. Diluting a well established brand by turning it into an 'umbrella brand' does nothing but erode its value and confuse the users. Once the damage is done, it can't be undone.