I'm not sure if anyone else has noticed this, but the Thunderbolt was advertised and sold to have 8GB eMMC internal memory + 768MB of RAM + 32GB memory card. While the amount of RAM and the SD card are correct, there has been some debate (elsewhere) that the phone only has 4GB eMMC internal memory, or that if does have 8GB, that a lot of this is unaccounted for.
The reason this has come up is that the phone only shows about 2.6GB of available internal memory. I've been told by a few people that a lot of this space is used by the OS and other apps, but I find it hard to believe that such a "high end" device can only have 2.6GB available (with 5GB being used). Remember, this is internal memory (eMMC) is the space where most all apps are stored, as well as other app data, emails, contacts, etc. Only some apps are written to be saved to the SD card, and pics/vids/music are also stored to the SD card.
What makes this even more interesting is that the Motorola DROID X has 8GB of internal memory, but about 6.5 GB of that is free.