Anyone betting that last week's 50-for-1 stock split of Berkshire Hathaway's Class B shares would lead to Warren Buffett joining the benchmark S&P 500 stock index .. is absolutely right and making some money tonight.

S&P says tonight (Tuesday) that Berkshire will replace Burlington Northern Santa Fe in both the S&P 500 and the more exclusive S&P 100.

Berkshire, of course, is acquiring BNSF in a deal expected to close next month.  When that happens, the railroad itself will no longer be publicly traded.

Not surprisingly, Berkshire B shares were up 8 percent in after-hours trading minutes after the news hit .. rising to $73.50 from today's 4p ET close of $68.06.

The higher-priced Class A shares were up 5 percent to $107,000 from today's 4p ET close of $102,010.

The big index funds that try to mimic the returns of the S&P 500 will now need to buy Berkshire Class B shares, boosting demand for the stock.

Longtime Buffett fan and value investor tells CNBC's Fast Money tonight that he's been stocking up on Berkshire shares because he expected S&P would tap the stock now that the split has brought its trading price closer to $70 than $3500.  (Tilson comments start about 2:15 into the clip to the left.)

He says he plans to hold onto the purchases because he expects the stock to rise even more because even after tonight's gain shares are still 20 to 30 percent below his estimate of the company's "intrinsic value."

And Buffett himself is probably smiling tonight.  During last week's CNBC live interview with Becky Quick, he sounded like he wouldn't mind at all if Berkshire made the S&P:

BECKY: There are some people who say, hey, this could mean that Berkshire Hathaway could now become a member of the S&P 500. What do you think of that?