I'd lean towards sticking with `rem` now.

IE7 and 8 support page zoom (http://windows.microsoft.com/en-GB/windows-vista/Zoom-in-on-a-webpage) - and it's not an accessibility failing of our site if users with visual impairments are using IE6 with no extra assistive technology to help them. If they are, it's reasonable to direct them to free Windows magnifying software to help.