Now that the State Quarter program is coming to a close, the U.S. Mint has a new plan for the $1 coin which will feature the face of a different president every three months. Read more here.
If you're keeping track, that means Nixon's coin will come out in 2016. That's the last president listed on their schedule. I think there may be some sort of condition or something where living presidents can't be put on currency or something like that.