Designer Sunman Kwon's "Mooon+" cellphone makes the most sense of any concept I have seen in a long time. Simply put, the mouthpiece detaches to become a bluetooth headset. Plus, both the headset and the phone look fantastic. This is definitely something I would like to see integrated into a real-world product. A near ideal solution. Additional pics after the break.
This is a nice concept, but I'd avoid it like the plague for exactly the reason Curves describes. Modular handhelds sound like a great deal, but they're not so great if you drop that piece and suddenly your phone is incomplete, suddenly lacking a mouthpiece to talk into until you can buy a replacement.
It's nifty, but stupid. Pass.