Situation: LLCP enabled Qt app running on the C7. Touching a second C7 works and both devices can establish communication sockets and transfer data.

Listening to the "com.android.npp" URI and touching a Nexus S enabled with the NPP (Ndef Push Protocol) should also establish an LLCP socket and allow transferring NDEF records. However, the Qt client socket returns: "an undefined error has occurred" (QLlcpSocket::UnknownSocketError).
-> First question: how can an error be unknown? Why isn't there an error code reported that is useful?

Most likely the issue is due to the specific LLCP socket parameters that need to be set. Unlike the Android NFC APIs, the Qt Mobility APIs do not seem to allow setting or modifying those lower-level socket parameters.
-> Interoperability between Symbian and Android LLCP should be possible.

NPP specs: http://source.android.com/compatibility/ndef-push-protocol.pdf

On the Android side, the service socket is created with the following parameters:
SERVICE_SAP = 16 (Service Access Point)
SERVICE_NAME = "com.android.npp"
MIU = 248 (Maximum Information Unit)
RW = 1 (Receive Window)
BUFFER_LENGTH = 1024

The connection-oriented client socket is created with:
SERVICE_SAP = 0 (Service Access Point)
MIU = 128 (Maximum Information Unit)
RW = 1 (Receive Window)
BUFFER_LENGTH = 1024