Ok, I got confirmation - this is an email that is automatically sent as soon as you access the service. It does not mean you are starting to rack up any charges, but that a billing account has been created so that when you do start building things, the metering will be activated.
To confirm no charges are accruing, every Virtual Data Center has a "Resource Usage" tab that is updated hourly. You should continue to see $0 until 1) You create a VM or 2) You add a public IP address to your gateway.
Hope that helps!
Jenny