I have no knowledge of what has befallen Senator Tim Johnson (D, SD), and there’s a disturbing vagueness to the lack of released diagnosis to date.
From the CNN article, the only part to describe the clinical picture:
Johnson spokeswoman Julianne Fisher said the senator was in the Capitol on Wednesday morning conducting a conference call with South Dakota reporters when “his speech pattern slipped off.” (Listen to Johnson’s difficulty speaking during a WNAX radio interview — MP3, 749 kb)
Fisher said the senator was able to walk back to his office in the Hart Senate Office Building, then began having problems with his right arm. He thought he was all right, she said, and went to his desk, but came out a few minutes later and “it was apparent he needed help.”
Several hours later he’s in the OR. Subdural hematoma springs to mind given the description of both a speech and one-sided arm difficulty, but only time will tell.
Update @ 8:31AM:
A U.S. Capitol physician says Sen. Tim Johnson underwent successful brain surgery for an arteriovenous malformation, a condition which causes arteries and veins to grow abnormally large.
That would do it, as well.