Nailed it there.

<Rather than pointlessly b1tch, moan and complain that
the n900 is compromised and not worth the effort>
An app could be created to address the issue.

I would guess that this could never be done completely
on an iPhone or an Android because backdoor comms
are probably invisible to apps inside their prisoncells.

The n900 could have just such an app to blockade
or at least inform the owner of any nasties being broadcast.

I thought wireshark would be able to show anything being sent,
perhaps there would be an easier way though,
since you would not necessarily need to listen to
anything other than outgoing messages.
For the truly paranoid it might be necessary to do some kind of
traffic monitoring on the inputs to the GSM hardware
to make sure there is nothing extra being generated
beyond what the system network actually generates.
Wish my broken unit was healed so I could check on this..