I thought it was at Griffith as well, but then I did a little more investigating myself:
April 14, 1910: With the weather perfect, President and Mrs. William Howard Taft, along with Veep Sherman, surprise the Nationals by showing up for the season Opener. Jimmy McAleer suggests Taft throw out the first ball, and he becomes the first president to do so. Walter Johnson catches it, then pitches the first of his 14 Opening Day Washington games, striking out 9. An easy fly hit by Frank "Home Run" Baker into the overflow crowd—a ground-rule double—mars his 3–0 pitching gem over rival Eddie Plank. Gabby Street is behind the plate for Johnson, the only opener in which he catches for Walter.
Taft became the first U.S. president to throw a ceremonial "first pitch" when he opened the 1910 season at Washington's League Park on April 14.
Let's see if someone gets if faster...if not, davidmarver gets it.