A client's Professional Edition org does have a (limited) Developer Console in it. I do know PE supports Visualforce. If I go into Setup and create a VF page there, I can save it just fine. However if I try to update the same page in Developer Console I get the error Can't alter metadata in an active org.
Now this is just strange, because Visualforce pages are never subject to this message in other orgs - only Apex. Moreover, even if PE supports sandboxes now, it has no Change Sets which means you still do have to edit Visualforce directly in the production org. Is Developer Console just the wrong tool to use for this in PE?