I just heard this on WTOP News Radio, so I don't have a link yet. The story is still coming out, but one witness posted a photo taken at the time. What looks like a muzzle flash can be seen near the balcony.
Update: Apparently the Secret Service is downplaying the story about the shots that were fired this evening. A Senior White House official said events got a little out of control when Deputy Chief of Staff Karl Rove told the President that 'Militants from the country of Snipe were invading.' The official confirmed the President then appropriated a firearm from one of his Secret Service agents and "went hunting."
Second Update: For those who didn't/don't get it, it's an April Fools joke about an April Fools Joke played on Bush -- Rove talked him into going to war and now Rove talked him into going snipe hunting -- which is actually quite believable.