Service Principal Names (SPNs) on Windows

[Markus Moeller]

When I read lately about setspn on w2k/w2k3 I noticed that the SPN can be 
service/host:port 
(http://msdn.microsoft.com/library/default.asp?url=/library/en-us/ad/ad/name_formats_for_unique_spns.asp) 
with a comment that :port can be used to differentiate between multiple 
instances of the same service class.
Does anybody know if this is only for non-Kerberos use ? If not how does it 
work with Kerberos ? Can I have two webservers on the same host listening on 
port 80 and 81with two different SPNs (e.g.  HTTP/host and HTTP/host:81) ?
I saw the port being used for SPNs in SQL setups too.

Thanks
Markus