We will get 2.1 when Google tells VZW they can release it.
From everything that I have heard, the N1 has been something of a boondoggle, with sales not where Google wants them.
Google has a vested interest in making sure the N1's sales increase, and that it's OS is stable. It's "their" phone, from what I understand., they had a major role in it's design. The Droid was designed largely by Motorola, so the profits from the Droid accrue to VZW/Motorola more so than they would from the N1. If 2.1 gets released to the Droid, by and large, an already stable device, with a good level of customer satisfaction, the consumer may decide that the Droid makes better sense, than the N1, therefore taking profits/sales away from Google. In my opinion, when they get the N1 stabilized, and the complaints pared down, and sales up, we will see the release of 2.1 to the Droid.....
Start thinking late spring, early summer.