Android disk encryption - using as a removable device
In the official HTC ICS ROM for HTC Desire S, it is possible to encrypt the SD card in addition to the internal storage. It seems likely that the same would be true for the HTC Desire HD. I haven't been able to find details of how this encryption is done, yet. 

With an encrypted SD card it is possible to mount the device as a removable disk when plugged in via a USB cable -- I do this myself for backups. 

I assume that the SD encryption uses the same passphrase as the internal storage encryption as no other passphrases are required. It could conceivably be done using some internal passphrase invisible to the user, though I would be surprised.