am2+ cpus only have DDR2 memory controller, and AM3 mobos use only DDR3 memory, so nope, it won't work that way... (and the sockets have slightly different pin arrays so it wont fit in, thou you can use am3 cpu in a am2+ mobo, since am3 cpus have both ddr2 and ddr3 controllers and the pins will fit this way)...
An AM3 CPU can fit and operate properly in an AM2+ board, but not vice versa. There are a lot of people who's confused about the backwards compatibility, and bought the AM3 board to go with their AM2+ CPU. Needless to say, they're infuriated when they found out that their $100+ AM3 board doesn't work.
Like previous posters said it won't work.Also if tried most likely you will destroy your AM2+ CPU and possibly wreck your motherboard.I've already seen someone who tragically tried to do this on the ASUS part of Tom's Hardware forum and wondered why he just got a black screen.
Here is the warning from AMD