The term Blue Moon can refer to the second full moon in a given month. When people say "once in a blue moon" they are speaking to the rarity of the occurrence of a second moon happening in the same given month. Thus the next scheduled blue moon – second full moon of a calendar month – will be August 31, 2012.
Can there be two blue moons in a single calendar year? Yes. It last happened in 1999. There were two full moons in January and two full moons in March and no full moon in February. So both January and March had Blue Moons.
The next year of double blue moons is coming up in 2018.