A article on the Nexus One Blog tells us that spare batteries will be available:
One of the great things about the Nexus One is that you can easily remove and replace the battery. If you would like to be able to carry a fully charged spare battery, we recommend official Nexus One batteries for performance and reliability.
You can purchase spare batteries here or by going to google.com/phone and clicking “Get your phone.” Then just scroll to the bottom to select batteries. Please note that there is a purchase limit of five batteries per customer.
They are $25 each plus shipping. Visit here to purchase. Has your Nexus on battery died? Tell us about it in the comment section.