Milestone actually started receiving the 2.1 update before the Droid because the OTA method used by Motorola/Verizon for the update was FUBAR.
Unfortunately, the different bootloader (Milestone requires signed ROMs from Motorola) and wireless hardware means the ROM for Milestone will most likely not work on a Droid.
You would probably have to root, and on top of that, the Milestone uses a different baseband, so a straight port is probably out of the question. We'll see though.
Was supposedly licensed third party software that enabled this feature, so "porting" it over would be illegal. Apparently, Motorola has only licensed the software for the Milestone and the Devour at this time.
Here's a thread discussing the issue on HoFo, starts around post #3: