I used to be pretty easy to disable on the OG droid, but can't seem to figure out how to remove it on the D3.
Once a root method becomes available you will be able to remove the startup sounds. On my Droid pro, the file that needed to be deleted was in system/media. This is also the location of the boot animation.
But the question is is root coming.
Moto has said they'll unlock all their phones in late 2011, but followed that release with "Where carriers will allow it"
VZW let's rooted phones on their system, but will they let Moto unlock their phones? Those are two separate things, one that worries me.
I'll be extremely surprised if it never gets rooted. Moto doesn't necessarily have to unlock the bootloader in order for root to be obtained. Look at the Droid X, 2, Pro, etc. We'll just have to wait and see.
On the Droid it was the SD Card mounting sound setting. Does the Droid 3 have that setting?
The setting was removed and the sound disabled on the Droid via OTA update.