If you're rooted, try this. Just tested it here and it worked. In Root Explorer, navigate to system/media/. Click "Mount R/W". Long tap on the droid.ogg file and rename droid.bak Tap "Mount R/O." Exit and reboot. Voila. No more annoying Droid sound on boot.
DISCLAIMER: This worked for me, but I'm used to doing a lot of hacking around in Linux. YMMV, but it's really simple.