Lately I experienced a problem with KVM-based virtual machine running a DHCP server and another one running a DHCP relay (for both I use the ISC implementations). The DHCP relay was complaining about "bad udp checksums". Using tcpdump and wireshark I quickly found out, that the software was right and the UDP checksums were in fact wrong. After some searching, I found a bug report, that basically described the same problem.
Although I cannot verify this, I think the problem might be related to the fact that I recently upgraded the host machine from Ubuntu 10.04 LTS (Lucid) to Ubuntu 12.04 LTS (Precise). As a workaround, I deactivated the use of the "virtio" support for the network interface in both virtual machines, which seems to fix the problem, because then the UDP checksums are correct.
However, when I performed the same change for a virtual machine still running on an Ubuntu 10.04 LTS host, this actually caused a problem: If VLAN interfaces are used inside the virtual machine, the normal non-virtio driver will screw things up on a Ubuntu 10.04 LTS host.
Long story short: For virtual machines running on a Ubuntu 10.04 LTS host you should use and for a Ubuntu 12.04 LTS you should avoid virtio networking.
Update [2012-07-08]: It seems like the conclusions I draw in this article are actually wrong. Therefore I posted an update clarifying the situation.