This bug appears to have been reported against 'rawhide' during the Fedora 14 development cycle. Changing version to '14'. More information and reason for this action is here: http://fedoraproject.org/wiki/BugZappers/HouseKeeping
I think it would be counter-productive to turn on vhost-net without also fixing Bug 642355. Without the new --checksum-fill rule in iptables, any RHEL5 / CentOS5 (as well as, I'm sure, several other OSes not yet identified) guest that had a virtio network interface would be unable to get a dhcp address. I'll leave it to someone with a higher level of authority to decide if this should be made into an official dependency in bugzilla, or just left as a recommendation in the comments.
This is fixed in already fixed in the qemu package in f16 and f17. Either way, the KVM package was merged with the qemu package, so any future issues should be filed against the 'qemu' component.