UPDATE: With the release of 10.7.3, you can now do this via "Server.app", so i'd advise against the below method. If you have used the below & upgraded to 10.7.3 & have an error in "Server.app" when managing the web service, click here.

Port redirection is (simply put), where an end user enters: http://mygreatwebsite.com & they are redirected to http://mygreatwebsite.com:(someport).

On Snow Leopard server you could easily set port redirects from within the Server Admin.app, but in Lion server this has all been changed.

The basics of how to enable virtual hosts on Lion server, is covered in this post. As well as links to the more advanced options.

Once Virtual Hosts have been enabled, editing /private/etc/apache2/extra/httpd-vhosts.conf enable the redirects. 

UPDATE: The below can now be completed using 'Server.app' & as such i'd not advise you perform redirects this way. If you have performed the redirects as directed below, you may have an error which has a resolution mentioned here.

With the move to Lion Server, Apple stripped moved administering the Web service from the new Server.app.

This over-simplification has caused me some head scratching as many of the options I’d become familiar with had been replaced with little more than an On/Off switch.

But don’t fret! It turns out that these options still exist, it’s just you need to hunt for them.

Virtual hosts are best explained in the opening paragraph here. To enable this on Lion Server do the following