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:
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).
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.