I'm bored out of my brain hanging around waiting for my bones to knit and ligaments to tie themselves together again.
So, FWIW, I went on with the bluetooth/iphone research. Everything about it is contradictory but this is as near as I can get to a consistent story and it might help someone someday.
Most commercial bluetooth GPS receivers use a standard bluetooth protocol called BluetoothTM Serial Port Profile - SPP, or sometimes called SPP for GPS. It emulates a serial cable to provide a simple realtime data transfer service. It commonly connects logging devices such as the GPS receivers, heart monitors, OBD scanners, etc.
As Suspense said Apple is right up to date with bluetooth releases, except that they only support a small proportion of the common bluetooth protocols, typically those associated with phone calls, address books, SMS and A/V apps. IOS does not support many/any(?) of the common data protocols. The problem is outlined here:
http://theiphonewiki.com/wiki/BluetoothApple lists the protocols it does support at
http://support.apple.com/kb/ht3647 SPP is not there.
Harry's lists the GPS receivers that do connect to iphones, etc at
http://www.gps-laptimer.de/Compatibility.html. Some use direct physical connection, Some use bluetooth and when they do the makers are very coy about the protocol used.
General forum chat supports the non-SPP issue and typically points to jailbreak and then installing roqyBT from.
http://www.roqy-bluetooth.net/wp/ if you want to use most GPS or heart monitors or similar scanners/loggers over bluetooth.
The reason for going to external receivers is sampling frequency. At 1Hz you can get lap times accurate to a couple of tenths but little else. At 5Hz you get lap times within a couple of hundredths of Dorian readings and more importantly you can pick gear change points, braking zones, racing lines, in-corner speeds, etc. AT 10Hz it's all just a bit sharper and you get much more useful acceleration data. With 5Hz and a phone you have enough info in your hand immediately after a session to work out where you went wrong/right and where you can focus your improvements in the next session. At 1Hz that would be almost impossible. Guran published some plots of 1Hz vs higher frequencies a while back, you'll see what it means.