I cant remember what they are called, but there are these strange sticky doo-dads that onlty stick when you want them too and they can hold quite an amount of weight.
stick em on the wall, then stick the speaker to the doo-dad and it stays put. When you are ready to take them down you pull the tab and the adhesive releases. Presto, it lets go without and residue or holes.
i had a buddy use these with a set of Boston Accoustic Bravos without any problems.
i wish i could remember what they are called, check home-depot or Lowes, i think they might have them.
they are not terribly expencive either.