Okay. Thanks to everyone that replied, it helped make this information quest a little easier. After numerous enquiries around the Internet, I think I have all the answers I need. If I've made any mistakes, please feel free to chip in. Mods, I'm not sure if this is worth a sticky, but it might help clear up some of the questions on here! Here goes.....
New 2008/2009 Models
These cars come with Bluetooth as standard. This means you can, without ordering any special extras, take any Bluetooth compatible phone and enjoy some level of functionality, be it simple hands free or extensive use of the cars controls and COMAND interface.
The level of functionality available for some phones and the standard Bluetooth fit is listed at this website:
Select your country, then car class, then "Bluetooth Connectivity", then "Compare All" to see a list of features
The Bluetooth protocol used for this standard Bluetooth fit is HFP. This simply means that your car will talk to your phone and share some of its features. Using this system, phone calls are made through the phones antenna, not the cars. The phone is effectively 'live'. This integrated system replaces the functionality of the Bluetooth HFP cradle:
If you wish to use the cars antenna to make calls (this way the signal is boosted) you can do one of two things. Either a) Buy the correct cradle for your phone (if it exists) - a decent list of available cradles can be found here:
or B) use a different sort of Bluetooth. By purchasing one of these:
your car will use the SAP Bluetooth protocol (be aware that there are two generations of this product. You want Version 2). The difference being that the car will take ownership of your phone, and place it into a hibernation state. The car effectively downloads all the information from the SIM card in the phone and uses it to conduct telephony. Again, be aware that not all phones support the SAP protocol. It is part of the Bluetooth Version 2 specification which is becoming standard amongst newer phones. Nokias seem to be the best supported. To the best of my knowledge, Apple hasn't adopted it yet. Another point to note is that irrespective of who makes your phone, 3G models will not work with the SAP protocol. They can still be used with the HFP protocol however - that is to say, the standard Bluetooth fit.
Both methods will allow the use of the cars antenna and maximise the feature set available to you in the car. This should include, amongst others, reading and replying to text messages on the COMAND interface should you have it. To view a list of features available when using a phone specific cradle, visit:
Select your country, then car class, then "Comfort Phone System", then "Compare All" to see a list of features.
To use either of the cradle types, you must have Optional Extra 386 - Telephone Pre-Wiring installed. Mercedes assures me you do not need any of the other options (388 - Universal Interface, for example) installed.