There's a lot more information on this Mazda forum:
http://mazda3revolution.com/forums/2010-12-mazda-3-audio-electronics/6131-updati ng-bluetooth-avrcp.html
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Here's a response from Apple that was posted to the forum
10-Jul-2012 02:19 PM Apple Developer Bug Reporting Team :
Engineering has provided the following feedback regarding this issue:
We discussed this issue with the manufacturer. They acknowledged there is a critical bug on their platform, as they require the phone to support no more than AVRCP 1.3. Because iOS supports AVRCP 1.4 since 5.0, they do not enable & use any AVRCP 1.3 feature. The bug has been fixed on their newer platforms.
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Also, this response from Mazda:
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...we have found that it seems that only phones with exactly AVRCP 1.3 are able to display the metadata. As you also stated, iPhones with iOS 5 and above use AVRCP 1.4, so they will likey not display the metadata. At this time, we have not been informed of any upcoming updates for the HandsFree system in your vehicle that would change the way the metadata functions. We apologize for any inconvenience.
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So, as noted on one of the more recent posts, the iPhone would need to send exactly AVRCP 1.3. I don't know if it is possible, perhaps through a Jailbreak app, to force AVRCP 1.3.
I'd say that overall, everybody with a stereo that only supports AVRCP (i.e. Mazdas) is probalby out of luck.