I copped the official explanation for the goal being disallowed from the NHL On The Fly post game highlights and looked it up. At first I didn't quite see it this way, but after looking at it this way, it makes sense.
As Ovechkin did not play the puck on the goal, but interfered with Price by charging and shoving Hal Gill onto him, the goal was disallowed in accordance to Rule 78.5 (v) of the Official NHL Rulebook. It reads:
78.5 Disallowed Goals – Apparent goals shall be disallowed by the Referee and the appropriate announcement made by the Public Address Announcer for the following reasons:
(v) When an attacking player has interfered with a goalkeeper in his goal crease.
What is most mystefying to many, is that if Ovechkin did interfere with Price as was judged, why was there not a penalty call? It's a good question, and the answer is a simple one. A minor penalty cannot be called once a play is over. In other words, since the whistle had blown and the play had ended, a penalty could have only been called against Ovechkin if an infraction had been signalled at the time of the incident.
Also curious is how a goal can be disallowed after an official points out that it is a goal on the ice. That answer is equally simple. The on ice official's only task in judging goals is to signify that the puck has indeed fully crossed the goal line. Again, in other words, what goes on beyond the parameter of the puck crossing the line can be brought into play after the fact.