NetworkManager is getting support for bridging. It is not yet in the master branch (as of 2012-06-14) and it doesn't yet work for me (pavlix) but I believe it's time to have some testing configuration to work with. It may not work for you, so feel free to modify this page when bridging support is generally available.
The 'keyfile' way
[connection] id=Bridge uuid=fc61f6e5-8900-4c6f-915c-eb55714eb874 type=bridge [bridge] interface-name=br0
[connection] id=Slave uuid=2e1906b1-92a4-4206-b2d3-eecfd07241e9 type=802-3-ethernet [802-3-ethernet] mac-address=52:54:00:8e:93:da slave-type=bridge master=br0
The 'ifcfg' way
UUID=fc61f6e5-8900-4c6f-915c-eb55714eb874 TYPE=bridge DEVICE=br0
UUID=2e1906b1-92a4-4206-b2d3-eecfd07241e9 TYPE=Ethernet HWADDR=52:54:00:8e:93:da BRIDGE=br0