There is an interesting article posted tonight at NeoSeeker. They bill the article as a review of Powercolor's X1800GTO card, but they test a number of different CrossFire configurations in an attempt to see if there is a difference in performance when using a dongle or not, and when switching from an x8 / x8 to an x16 / x16 PCI Express Lane configuration.
"Our Crossfire testing proves that two X1800 GTO's, even on an older dual 8x board will still provide good performance. Despite the gains that an X1800 Crossfire Master card setup present, that's hardly a viable solution and should not be considered in place of another X1800 GTO. Hopefully ATI can extend the cableless option across most of their range to make their dual-GPU solution truly competitive with SLI as far as usability goes. While bandwidth would likely be a limiter on higher end boards, any solution ATI can work out that doesn't require the cable will be a bonus to them and the user."