GPL client locked into closed source backend - is this really GPL?

[Tim Shephard]

Hi Folks,

If you have a client-server application and GPL the client but it has
a very broad and very tight protocol coupling with a backend server
that is closed source .. is that really GPL?

One example of this is Second Life, where the client speaks a very fat
protocol against a backend server which is closed source and highly
patented.
I am sure there have been other examples though.

Any opinions?

Cheers,

Tim.