Title is question. THanks!
http://community.vzw.com/t5/ThunderBolt-by-HTC/WHAT-COVER-CAN-I-USE-NOW-THAT-I-HAVE-AN-EXTENDED-BATTERY/td-p/484236
You can use a silicone, but it looks horrible. I'm sure they are working on one due to the Thunderbolts grossly undersized factory battery.
I found a great site that offers hard rubber cases for the HTC Thunderbolt w/ extended battery, I received mine and it fits like a glove. They are only charging $14.99 for their cases. Hope this is what you were looking for!
http://customhtccase.wordpress.com/
There is a guy with userid moretalktime on eBay that sells a pretty inexpensive high quality TPU case that allows for use of the kickstand too.
I have a Seidio case for the extended battery. Found the best price at http://www.cellpower.com