There was a day when any running instance of certain Microsoft products, such as SQL Server, suffered from license violation if that workload was migrated to another host inside of a 90 day period. That is, Microsoft would allow a customer to migrate SQL Server to a new host, but not more often than every 90 days. Under old rules, SQL Server had to be licensed as if it were running on every physical processor in a cluster. So, if you had ten dual processor ESX hosts, you would have needed twenty per processor SQL Server licenses.
Under the new licensing rules, you need only to license those physical processors that will actually support the virtual workload.
Microsoft has created a comprehensive document detailing the new licensing rules. This document explains fully what restriction there are in this program and what you need to do to ensure that to remain complaint: Microsoft Volume Licensing Brief: Application Server License Mobility