This is very hard to do as Motorola requires a signed kernel in order to boot. They are currently considering an upgrade, but haven't decided yet. O2 in Germany claims to be able to push it out by mid September. if this the case, then I would just wait for it.
Of course, information on HOW to do it would be nice as well, in case Motorola doesn't release it here.
The Droid is a CDMA phone, while the Milestone is a GSM/UMTS phone. The radio drivers from Droid will not work on the Milestone. Also, there's the issue of the locked bootloader on the Milestone as well ... I doubt the ROM will boot up at all.