I couldn't get network bridging to work on a Dell Poweredge 850 with the inbuilt nic (Broadcom BCM5721) on Ubuntu intrepid.
The bridge was created fine, brctl showed eth0 joined properly. No iptables on host or guest. Routing rules correct.
The behaviour was very similar to this bug. When using DHCP, the vm guest would make DHCP requests that reached the DHCP server, but never get any replies sent by the DHCP server. If a static IP was set the box continually arps without receiving any of the arp replies being sent by other boxes. If you look at the arp cache it shows all entries as incomplete.
I eventually gave up, copied to different hardware, and it worked perfectly - same vm, same xml definition, same version of the installed host OS.