Engadget has a nice thread on this, referring to the Kodawarisan site. They seem firmly convinced that magnets are used, and that they would not cause problems with the HDD, etc.
What I want to know is whether the VESA adapter for the 2005 G5 iMacs can be attached to these, similar to what was discovered for the 2006 intel iMacs. It would require tearing down the computer to get to the boot hinge, which could be a bit daunting, but if so it provides those who need to mount the computer on LCD arms the option of upgrading to the newer iMacs.