I've been shopping for a Droid so I can finally get some Cyanogen, I have a DX.
To satisfy the curious cat, I shot an email to the VZW rep I got my DX's through. She said though they don't have them in stores, you can purchase Certified Droids through VZW at 289.99 a pop. So apparently they still have them and I would imagine for quite some time, your Droid will be replaced with another if under warranty.
The Nexus One would never be a replacement as it was not produced with the CDMA and EvDo radios, to my knowledge, that would be needed for Verizon. Not to mention it is a Google phone not a Verizon phone.