That hasn't dampened investors' enthusiasm for the company's stock, however. It surged 24 percent today.

US Bancorp is probably the least likely of Buffett's banks to be ordered to issue new stock or convert the government's $6.6 billion from its Troubled Asset Relief Program in to common equity as a result of the stress tests, according to analysts. It reaps more income from fees than many of its competitors, providing it with a more stable stream of revenue. Its shares rose by over 13 percent today.

Of the three banks winning Buffett's seal of approval, only M&T has outperformed the KBW Banks Index of large financial institutions so far this year. Coincidentally (or not), it's also the only one not subject to the stress tests. Perhaps the best way to avoid a bad grade is not to have to take the exam in the first place. M&T jumped almost 14 percent on the day.

But Buffett isn't as enamored of all of his bank holdings. SunTrust (STI), another Berkshire Hathaway investment, is posting big gains today -- it climbed 25 percent -- despite his refusal to offer the same endorsement of its financial health he gave Wells Fargo and US Bancorp.

"I don't know that much about SunTrust in order to give you an answer" as to whether it will need to raise capital after its "stress test" is graded, Buffett told reporters yesterday, according to Bloomberg News.

Notwithstanding Buffett's uncertainty, analysts who follow SunTrust have said the company may well be among those that regulators ask to raise additional funds. Oracle, indeed.