There was one tie before in 1979 in the NL between Keith Hernandez and Willie Stargell.
A tie should be relatively hard to pull off because of the balloting method. Each voter is required to rank 10 candidates. Points are awarded in this manner: 14-9-8-7-6-5-4-3-2-1. And with the possibility of three or four players receiving first places votes, everything would have to break just right to get a tie.
Personally, I believe any ties should be decided in the House of Representatives, or, alternatively, by the use of odds and evens.
The closest AL MVP race was in 1947 when Joe DiMaggio beat out Ted Williams by one point, but a different ranking system was used then. Under current voting rules, the closest race was in 1996 when Juan Gonzalez edged Alex Rodriguez by three points.