Linux would still be the default kernel for Fedora, but a FreeBSD kernel would be a new option for those enthusiasts wishing to switch to the FreeBSD kernel in order to use DTrace, its ZFS file-system kernel support, or other BSD functionality. The GNU user-land from Fedora would remain the same.
This Fedora-on-FreeBSD would be similar to the successful Debian GNU/kFreeBSD operating system or the still yet to bear fruit alternatives: Gentoo FreeBSD and the recently covered Arch BSD. There's also Starch Linux that attempts the opposite: OpenBSD while having Arch Linux underneath.
About this F19 proposal, the Fedora Project Wiki says, "This feature proposes that Fedora userspace will be ported to run on top of FreeBSD kernel. This Feature will provide more Freedom to lot of Fedora users and Friends. Our target is to provide state of the art integration of our userspace components with FreeBSD kernel. As a side effect, Fedora will maintain it's First position among other distributions (eg. Debian GNU/kFreeBSD, Arch BSD, Gentoo FreeBSD) which are already working on their own FreeBSD kernel ports."
Giving this proposal some weight is that it's actually backed by Red Hat employees. Jiří Eischmann, Pavel Šimerda, and Martin Holec are the ones wanting to work on this BSD kernel for Fedora so that it gives users the freedom of picking the kernel best for them.
It will be interesting to see if the Fedora Engineering & Steering Committee approves this as an official work item for Fedora 19, which is due to be released in May. If it gets the go ahead, it will likely be a release or two before all the kinks of a FreeBSD kernel in the Fedora world are ironed out.
Update: Jaroslav Reznik, the Fedora "Feature Wrangler", has written in to clarify the situation. "...it's really a bad joke of one Fedora contributor. Even guys written on the page were not aware of being put on the page...I've immediately switch the page to a joke category."