Is Port Number Required in HTTP "Host" Header Parameter?

UPDATED for modern day browsers:

Browsers (and curl) will add the port only when it is not the standard port, as required by the HTTP spec and noted in @superfell's answer.

Browsers this day (2013), will actually strip the port from the Host Header when the port is the standard (http port 80, https port 443). Some clients, which use their own method, like the Baidu Spider, include the port number even when the port is 80.

Whether this is proper or not, I don't know. The spec doesn't say whether it's OK or not to include the port number when the port used IS the default.

To answer your comment, servers will do whatever they need to do to comply with the spec, and the spec suggests only the cases WHEN it's needed. Because of this, I feel It's not really a question of how the server deals with it - it's more how the client issues the request: includes the port number in the Host Header, or not.

See section 14.23 of the HTTP spec which specifies that the port # should be included if it's not the default port (80 for HTTP, 443 for HTTPS).