I was in a Verizon corporate store today and bought the car dock and wanted to get the extended battery, but it is at least twice as thick as the original and you need a new and bulky back cover and it no longer will fit in the car dock. I think I'm just going to get a standard OEM replacement battery.
But how will you charge the spare? I ordered a charge with 3 replacement batteries from ebay. I may get an OEM battery and use the charger.
3x BATTERY+DOCK Charger For HTC Thunderbolt 4G VERIZON - eBay (item 330541582074 end time Mar-23-11 03:30:44 PDT)