No there is nothing in licensing term you need to worry about with EWS its just viewed the same as using any of the other protocols eg IMAP, POP or Mapi as long as you are accessing an Exchange Server (this is the part that's important if you
used EWS for a purpose other then this you maybe violating a patent) . The only place where licensing would come into would be from a CAL level at the server eg to access mailbox you need to authenticate to Active Directory
http://www.microsoft.com/licensing/about-licensing/client-access-license.aspx which would require a CAL. The Mailboxes your application is using if they are dedicated to your application (and not user mailboxes) then they would require an Exchange CAL.
This is all really just standard licencing that affect any applications that connect to Exchange so there is nothing specific in regards to EWS or Exchange.
Cheers
Glen
