I've been trying this for a few days with no success.
I tried installing the rawhide fedora-release package and also tried disabling all the repos except the devel ones but I get errors about conflicts regarding kudzu and initscripts.
I've also tried the instructions here -
http://fedoraproject.org/wiki/Docs/Drafts/TestingGuide
but when I try to install the kernel I get even more dependency issues/conflicts.
I've even tried upgrading a few packages at a time but still get to a point where there are conflicts and can't get any further.
[Edit] Turns out I must have missed out the rpm -e <kernel version> from the Testing Guide since I still had an older kernel installed. Once I removed it everything upgraded without a hitch.