The iPhone Bluetooth currently only supports telecom headsets and, by proxy, compatible Bluetooth 2.0 EDR in-car handsfree systems. And to have in-car systems support 'Visual Voicemail' per se, would require both support from Apple and the in-car system designers (auto makers). Apple would need to support the playback of the audio files through the Blutooth interface. The in-car system designers would need to add support for the controls necessary to call up the visual voicemail files for playback. Otherwise the user would need to maninpulate the controls directly on the iPhone, which would defeat the basic purpose of the handsfree system.
There is a workaround, however... You can access your voice mailbox as a normal call, just like you would with any cellular phone. Just call your cellular phone number, enter your mailbox password, and retrieve your mail.
Not as cool and direct as visual voicmail on the handset, but it works.