After seeing the Ultimaker Belt Tensioner by psync (http://www.thingiverse.com/thing:10034), I decided to model my own that didn't need a screw. But of course there's also Roboteer's Paperclip belt tensioner (http://www.thingiverse.com/thing:4535)
Note: It's been mentioned in the comments that there isn't enough space between the new style Ultimaker slider blocks and the belts. Which causes the tensioner to hit the blocks.
Hook the tensioner onto the belt (as per the picture) in such a way that it won't interfere with movement.
If you need more tension, make the part thicker (or make the post width thicker). Need less tension, make the part thinner
If it's too difficult to hook on, make the part longer.