The OSI mailinglist has a discussion going on how a Corba interface to a GPL application can circumvent the derived work clause.

I am concerned whether a Corba interface can be used by non-free software to
circumvent the freedoms and requirements of the GPL license. […] A proprietary vendor could create non-free software that functionally would amount to a derived work, without actually making a derived work within the meaning of copyright law. Would this break the spirit of the GPL while complying with its terms, hence not be enforcable under copyright law?

As I understand, this discussion is in fact about any service aka RPC API like SOAP, XML-RPC. Hence, the essential question is whether Web Services form a potential thread to the GPL.