Is Mr. Rosenberg defecting from the dark side? Not quite yet, he says. But he maintains he now is more optimistic than he has been in more than a decade.

"I'm so excited I just can't hide it," Mr. Rosenberg wrote. "But for now, I'm keeping the powder dry."

Yes, Europe is in the midst of a major crisis. Yes, the U.S. has big, structural problems. However, while most analysts are just catching up with those realities, Mr. Rosenberg has been digesting them for years. Now, he is looking ahead and feels that the long-term outlook is bright.

"It's epic," says Russ Certo, who runs bond trading at Gleacher & Co. and has followed Mr. Rosenberg for 15 years. "To see Rosenberg change his tune, it's a wonderful piece of literature."

Mr. Certo says he, too, sees the parting of the clouds that Mr. Rosenberg sees.

"Just having something positive to say has elicited a huge reaction," Mr. Rosenberg says of the response to his note.