The Jabra SP200 Speakerphone for iPhone 3G/3GS [$59-95 - iMore Store link] is a great Bluetooth headset alternative that you really have to try yourself to fully appreciate. I can not recommend it enough.
I've been using the Jabra SP200 for about a month now and I can honestly say that I've stopped using every single Bluetooth headset I own while driving my car. Simply connect the SP200 to your cars visor, flip the power switch and you are good to go. And go you will - Jabra claims the battery life comes in at 10 hours of talk time and 23 days of standby time. I am happy to report that these claims are dead on accurate.
Now to get to the most important part of a Bluetooth device, sound quality. I was pleasantly surprised on how well not only the person I would speak to sounded, but how well I came across to them as well. Volume is definitely not an issue with the SP200 as not once have I had to turn it up to it's full capability.