Configuring an approval workflow to request approval from a group of approvers and deem the workflow complete after any single person has completed their approval task is trivial but it's not that obvious. I certainly missed it on my first pass through today!
The Assign tasks to radio button options on the workflow customisation page focuses on the parallel versus serial distinction. If you've got multiple people configured as approvers, you're either assigning tasks to everyone in the list or you're assigning tasks sequentially to everyone in the list; in both cases every approver has to complete their task before the workflow is deemed complete.
In the past, we've noticed a behaviour where approving a page doesn't seem to do anything—the page status stubbornly remains as Waiting for approval no matter how many times or how many different ways you attempt to approve the page and the fact that you’re a site collection administrator web application administrator, and farm administrator makes no difference! (Without email configured on the server and perhaps some SSPs bits in place, as was the situation in our case, no one will ever know additional input is required unless you're checking the workflow status). In all likelihood, the root cause of this "problem" was an approval pending completion by virtue of being assigned to twenty people. Doh!
But I digress. Adding approvers to a dedicated Approvers group is the logical thing to do in most cases to simplify approver management. Instead of changing multiple workflows as employees come and go, it's much easier to manage a SharePoint group or an AD group. Selecting the approvers list is easy but how do you get the "any approver" bit happening?
The answer lies in the check box directly below the Approvers selection input field: Assign a single task to each group entered (Do not expand groups). Think about it long enough and it actually starts to make sense—only one approver needs to complete their task before the workflow is deemed complete, or to paraphrase: “assign the task to the Approvers group and no one person in particular.” This option isn't selected by default so select it and you're golden.