Engadget writes about the complexity of just getting Bluetooth headsets to work with Bluetooth cell phones: Can I second this emotion? Bluetooth has two different kinds of remote talking: headset and hands-free. Some phones support one, some the other. Bluetooth in cars uses hands-free, but with some phones not supporting that mode, they won't work with the vehicles.
(Bravo, say I. Studies have popped up all over that show that it doesn't matter whether or not you're hassling with the cell phone to reduce your awareness of the road and increase the incidence of accidents. No talking while driving, please, says this bicycle commuter who has frequently almost been hit by cell-phone-enabled drivers--with headsets!)
A related woe for Mac OS X users: the OS supports Bluetooth extremely well, and you can use Bluetooth with your cell phone to send SMS, shunt people to voicemail, and to dial. The remarkable Salling Clicker lets you trigger all kinds of operations from and to cell phones, including using a cell phones joystick selector to move a mouse, or using a cell phone's proximity (entering/leaving) to lock a computer down or set an instant-messaging status note to available/away.
But if you use your cell phone with your Mac OS X system, you can't use a Bluetooth headset. You have to turn off your OS X Bluetooth connection to re-enable the headset's connection. Even more complicated is using a Bluetooth headset with Apple's iChat, support for which they added recently. The headsets can only pair with a single device. Since the Mac can't shunt sound (yet?) from the headset to your cell phone, you have to repair back and forth to use the headset with iChat and your cell phone.
Bluetooth: It's so simple, even a child could swear at it.