The first thing I thought of was magnets. Just buy some magnetic strips and cut them to size, get yourself some good sturdy glue, and go go go. You could buy a magnetic bullitin board (This one
is $20, but I'm sure you can find one at Target or Wal-Mart cheaper) and then you can spell out whatever kind of message you feel like at the moment and it'll be easy to change later!Here
is a link to magnetic paint if you want to magnetize a Scrabble board, wall, or whathave you. I don't know what color it turns out to be - maybe it'll make the Scrabble board gray?
If you do this I'd test it somewhere first to see the color.
is a link to another craftster.org project where they used the board as a regular bullitin board and put tacks on the back of the letters to make them stick to it.