I'm seriously considering a Canon 24-70 f/2.8L as a "general use" lens (I've been using a Sigma 17-70 f/2.8-4.5 with a Rebel XT but recently upgraded the body to a 50D. In time I plan on upgrading to a full frame body such as the 5D Mk.II or Mk.III, or in addition to the 50D, keeping the latter as a back-up body) but have followed the rumors of a new 24-70 possibly arriving at some stage.
Don't buy a 24-70 while right now you have a 50D with a Sigma 17-70.
You are seemingly still in doubt about buying a 5D MKII or MKIII , so why hurry to get a 24-70 MKI right now...
Dont bother about prices might raising on the 24-70 MKI , when the new 24-70 MKII hits the market , there will be many owners selling the 'old' version in order to get the new version.
Since so many 24-70 MKI were sold I dont see the problem to hurry and buy a lens now for a camera you will own later.