UPDATED Feb 5th:
Just noticed that if you install SP1 for WSS there is a new
STSADM command called renamesite.
Check out the details here:
http://technet2.microsoft.com/windowsserver/WSS/en/library/aeb73d9b-1734-4a62-9bbb-f092ee4f58e11033.mspx?mfr=true
This seems like it should be an easy task, but always makes me
think for a few minutes before I work it out. Here are a couple of
simple steps to show you (and me) how its done.
1/ Open up central admin and browse to 'Alternate Access
Mappings' found under the operations tab.
2/ Click the 'view' drop down on the top right of the URL list
and select 'Change Alternate Access Mapping Collection'. Then
select the web application with the URL you would like to
modify.
3/ Select 'Edit Public URLs' and change the URL's for any zones
necassary. Although the concept of zones is more semantic than
anything, they do represent the different URL's a web application
may be accessed by (and should suggest a use eg intranet zone for
an intranet web app).
In my example I am changing http://wsstest to http://wssfun.
4/ The final step is to update IIS to reflect the changes. Open
up IIS manager (start | run | inetmgr) and browse to the web site
that corresponds to the zone you updated. Right click the
properties, select the web site tab and click advanced. Now change
the host header to the new value.
The web application should now be using the new URL.