I posted this in the cases thread and figured I'd post it here too...since you asked.
I have been able to get the seidio surface case to work with the extended battery!
Two things needed to be done for it to work.
1. Peel away and cut off a little of the felt backing inside the bottom portion of the seidio surface case.
2. On the standard battery cover (not the extended battery cover), snap off the two tabs on the bottom that snap the cover into place on the bottom of the phone. This will allow you to use the standard battery cover with the extended battery.
Now put the seidio surface case back on and you're good to go. Hope this helps some wanting to use the extended battery with a case. This method may work for other cases too. Post your results if you give it a try.