even visitors with BBs bought elsewhere will be unable to use most services
Yes, that's my understanding too. Just to get technical a second - BB devices use a bespoke connection to the internet (not the standard internet connection that your phone has, do a search on your access points, and you will see one called "Blackberry" or "Blackberry2"). This is the secure VPN-type connection to the outside world, that skirts round the Etisalat and du folks. This connection will be barred from the radio system come 11 October, so it is not going to matter one jot whether you bought your phone in the Mall of the Emirates, the Mall of the Americas or Carphone Warehouse in Scunthorpe. Your BB will not be working post 11 Oct.
Other messaging systems, like Nokia Messaging, use the standard internet connection access point, which means they go via the local servers, and can be read.
My guess is that we are going to see some back pedalling by BB - this could well snowball, as people in other countries realise that BB allows some countries to read the lot.
For the record, I can sort of see the UAE point. There is something a bit odd when a government cannot read its citizens' emails, but another government can. I don't actually approve of anyone reading mine - they'd probably die of boredom - but if someone is going to read them, I would have thought the government of the country I live in, work in and party in has more right than someone in the US....
TA