I'll have to check my old OG Droid when I get home. I know it still shows the signal strength (and sync's time oddly enough) but I don't know about showing the old phone number. In truth, it probably doesn't as I never activated it with a carrier after the I last flashed a rom.
I would do a factory reset and wipe data and cache before you try to sell it (good practice anyway). When it reboots, bypass the activation and make sure your phone number is cleared out. If it's all set, factory reset again, and when it boots to the activation screen, turn it off and it should be ready for a buyer.