It's not for Bluetooth and Bluetooth isn't a company that VWoA failed to contract with.
The button is for switching the entertainment system over to telephone control in cars that have been equipped with the wired telephone 'prep' kit. Some US and Canadian MkV Jettas had it when they arrived in port and some dealerships were installing it too. It was (and may still be) an option.
However, the prep kit comes incomplete and requires the owner to invest in an additional cradle to match his/her phone. They're largely Nokia models but there are a few Motorola, Siemens, and Sony Ericcson models supported too. I've found at least one oem supplier with compatible cradles at nemesisgb.com, but they do not have a cradle for my phone... which is why I'm going with a Motorola IHF-1000 Bluetooth connection. If/When I trade in my phone for another, I won't have to source another cradle for a wired kit.
Now, for the steering controls, I'm finding some very promising things. Using the line-level input pins on the radio (telephone only guys, sorry) the volume control for the Moto kit is redundant; the radio controls the volume level of my phone calls. Same with the mute button (the star key). All I really need is the hook button... and that will substitute for the Moto kit's big blue button quite nicely to make or take calls and surf the setup menus (by voice).
