With any deployment in Windows Azure, a single public IP address, known as a virtual IP address (VIP), is assigned to the service for the customer to use for all input endpoints associated with roles in the service. Even if the service has no input endpoints specified in the model, the VIP is still allocated and used as the source address assigned to outbound traffic coming from each role.
Throughout the lifetime of a deployment, the VIP assigned will not change, regardless of the operations on the deployment, including updates, reboots, and reimaging the OS. The VIP for a given deployment will persist until that deployment is deleted. When a customer swaps the VIP between a stage and production deployment in a single hosted service, both deployment VIPs are persisted.
A VIP is associated with the deployment and not the hosted service. When a deployment is deleted, the VIP associated with that deployment will return to the pool and be re-assigned accordingly, even if the hosted service is not deleted. Windows Azure currently does not support a customer reserving a VIP outside of the lifetime of a deployment.
More information on the Windows Azure VIP can be found here.
Corey Sanders, Principal Program Manager Lead, Microsoft