Blockbuster, Zappos, and AOL have all been burned by having language in their ToSes to that effect. It's a bad idea if you want any legal cover from your TOS. IANAL, but this guy is and he says

  STOP PUTTING CLAUSES INTO YOUR CONTRACTS THAT SAY YOU CAN AMEND THE 
  CONTRACT AT ANY TIME IN YOUR SOLE DISCRETION BY POSTING THE REVISED
  TERMS TO THE WEBSITE

http://blog.ericgoldman.org/archives/2009/06/stop_saying_we_...