To my knowledge, you cannot reserve the IP address.
The way to handle this is to register a CNAME record in the DNS instead of an A (address) record. In other words, your custom DNS name should point to <service name>.cloudapp.net instead of pointing to the IP address. Then, the Azure infrastructure
will handle resolving <service name>.cloudapp.net to the Virtual IP address where your service is at.
There was a recent change in the documented permanence of the VIP address. The Windows Azure team blog post announcing this change
-- 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.
As Vidar points out, the solution is to use a CNAME record.