Just as we knew from Verizon's training procedures, the HTC ThunderBolt will be able to support both voice and data at the same time. In a leaked document from Verizon, we are given confirmation that the ThunderBolt will be one of the devices that features simultaneous data and voice.
They go on to mention that this feature isn't advertised because it's not common across all of their devices and they're not in the position to promise it will be consistent with their branding.
Hopefully Verizon will feature this functionality in most of their upcoming 4G (and 3G?) phones, we can hope - right?