Is there an easy way to diversify your Bitcoin holding among the other popular forks?
Sidestepping the comments about whether or not Namecoin is a fork or a separate project or whether bitcoin is an investment or a currency, I'll address the direct question asked: Is there a way to easily and automatically purchase several crypto-currencies at once? Not at present time, no.

Bitcoin (and its forks) is a very young project, though, and I'm sure there was a stock exchange before DJIA or S&amp;P existed. Give us time and it's very likely that something of the sort will be created - especially now that the demand has been made publicly known.