QDBusAbstractInterface::isValid() returns true only if the currentOwner is not empty, which will never be the case for a p2p connection.
I would like to propose two changes:

1) For p2p connections, completely ignore the service parameter in the QDBusAbstractInterface constructor (currently, if a service name is given, the QDBusAbstractInterfacePrivate constructor will call getNameOwner on the connection, which will fail).

2) For p2p connections, make isValid() return true if the connection is connected and the isValid private member is true.

I could try to provide a patch if desired (and I wonder, could we get this to Qt 4.8 as well?).