Microsoft licensing for IIS is interesting. My understanding is that you only have to pay a license fee for IIS if you are using Windows Authentication in IIS. If you’re using Windows Authentication, you may have to buy CALs or an External Connecter License.
In summary, it would seem that you don’t need to buy any additional license (other than the operating system license for the server) for IIS, unless you use Windows Authentication. If you use Windows Authentication, I suggest that you contact a vendor to discuss licensing. Many large software vendors (I’m not talking about high street stores here) have Microsoft Licensing specialists on hand that can advise you in full of this year’s madcap license schemes Microsoft dreams up :p