As GPS software becomes increasingly common for the iPhone, developers will continue to release car mounting solutions designed to help improve the GPS viewing and hearing experience.
Kensington’s Windshield/Vent Car Mount with Sound Amplified Cradle for iPhone ($40) doesn’t have the most pithy of names or any special antenna built in to improve GPS reception, but it does use a redesigned iPhone-holding cradle with a passive sound amplification system that promises to double the volume of an iPhone, iPhone 3G, or 3GS when placed inside.
You can attach either a windshield mounting pipe or a vent mount to the iPhone cradle, either likely to move the device closer to your car’s windows where the GPS reception should naturally improve.