Warren Buffett's Berkshire Hathaway (BRK.B) was added to the S&P 500 Index at the close of trading Friday, meaning millions of retail investors already have -- or will soon enough -- a little piece of the legendary investor's company to call their own.That's because about $1 trillion worth of index fund assets track the S&P 500 ($INX), the most common proxy for the broader U.S. equity market. Since most folks own stocks through their retirement plans, and S&P tracking funds are a core holding of so many of those plans, chances are you're already an owner of Berkshire Class B shares. (Most big S&P index funds are allowed to buy new entrants ahead of their actual inclusion, and if they haven't yet, they will: If a fund mirrors the broader market, it will have to own Berkshire Bs.)

Prior to a 50-1 stock split last month, the $3,400-and-change share price of the Class B shares precluded them from S&P 500 membership. The stock, currently trading at almost $76, has gained about 10% since the Jan. 20 split, helped by index fund buyers.

Berkshire introduced the (relatively) lower priced Class B shares in 1996 in order to allow smaller investors to gain entry to the company. The Class A (BRK.A) shares, which won't split, currently go for about $114,000 a pop.

Buffett maintained the high prices of both classes of stocks to keep out "inferior buyers," since the cost of a single share made the stock too unwieldy for professional speculators and too pricey for most retail investors.

The split was necessitated by Berkshire's November agreement to buy the 77% of Burlington Northern Santa Fe (BNI) it didn't already own. Berkshire is paying $100 a share for the railroad, with 60% in cash and 40% in Class B stock. By splitting the stock, Berkshire is essentially breaking a large-denomination bill into a big pile of small change to distribute in the stock portion of the purchase.