Can't tell you yet but I'm setting up KVM on ubuntu 8.04. I've installed bridge-utils 1.2-2 (as part of installing kvm 1:62+dfsg-0ubuntu8.2) and uml-utilities 20070815-1 (required for the way VirtualBox' uses bridging).
This is working OK for VirtualBox.
Now I'm working out how to set up a KVM virtual machine and install an OS. I will tell you what I learn regards bridged and tap networking when I get there.
In case it helps, my /etc/network/interfaces file is posted in this LQ thread
. It is for VirtualBox but may give you some clues.