I did read somewhere that when setting up an Ethernet bridge the system acting as the bridge doesn't show up on the network by default... Also if one of your interfaces is a wireless NIC bridging probably wont work.
I will go and see if i can find the tutorial...
Did you try ifup br0?
EDIT:
I found it, look
here
look at step 5 in "6.2. Basic Setup"
Hope this helps