Tim Bray wrote an interesting post on using POST to initiate an action; interesting comments and an illuminating post by Roy Fielding followed. My favorite comment comes from Andrew Wahbe:
Worrying about the way you've structured the resources on the server is usually a sign that you actually aren't thinking RESTfully. Why? Because you should be worrying if your hypermedia format frees the client from any dependencies on that structure. If you've got that right (and most haven't) then you can worry about using the method with the strongest semantics for each operation (GET for idempotent and safe operations, PUT or DELETE for idempotent and un-safe operations and POST for everything else). That will allow you to tune the plumbing (e.g. caching) as needed. Anything else is a waste of time. Futzing with resource structures doesn't yield practical benefits and doesn't make your system more RESTful (despite popular opinion).