There are two things which I don't understand:
1) Buffet usually buys companies, or at least a controlling stake in his companies. He doesn't like someone other than Berkshire managing his investments.
2) 10% sounds like a lot, but Buffet normaly doesn't invest in things which yield less than 20%. These yield are hard to come by at the moment, but normally he would just keep sitting on his cash. The reason they have so many cash available at this moment is because he hasn't been able to find such sweet deals in recent years.
Maybe, and I am out on a limb here, there are other interest playing. In his annual letters he mentioned a couple of years ago that he had a sizeable toxic derivate position via one of his holdings, which he wasn't able to offload.
From his letter:
But closing down a derivatives business is easier said than done. It will be a great many years before we are totally out of this operation (though we reduce our exposure daily). In fact, the reinsurance and derivatives businesses are similar: Like Hell, both are easy to enter and almost impossible to exit. In either industry, once you write a contract – which may require a large payment decades later – you are usually stuck with it. True, there are methods by which the risk can be laid off with others. But most strategies of that kind leave you with residual liability.
Could this deal also mean that he quietly could get rid of these investments via the backdoor? In that case the profit for BH would be much more than just the profit on GS stocks.
I guess the FED would also like to keep this quiet. It is one thing if some greedy bankers get burned. It is another thing if the most valued investers fall for the same trap.