Wipe data/factory reset.
You'll have to enter your account info again, and will lose your contacts, texts, and app downloads (paid apps will still be recorded in the downloads of the market I believe). If your contacts are all google contacts, they'll come back when you sign into your google account.
Edit: Also, don't be surprised if you get an email congratulating you on your new Nexus 1. Apparently Google has an auto-send email to thank people for buying the Nexus whenever someone signs onto the market from an Android 2.1 device, since it's the only 2.1 device at the moment lol