Had this wildest thought about mobile phones on the way home from the gym.. most due to my ..er.. disappointment with my Siemens S40.
First.. start by eliminating the screen and the keypad. Assuming I won't be making any calls where a normal talking voice is too loud, I much prefer voice-activating my phone, either by preprogramming the speed-dial, or just dictating the phone number. This way, the phone unit will only need to be as large as the battery and the circuit-board (remember that the circuit board will be smaller too now that the parts responsible for display and the keypad is gone too). Now, put the power and activate buttons onto the headset. So the base unit is as small as possible, almost wearable, and the headset can be embedded into appareals like a baseball cap for headband. Of course now the headset will connect to the base unit with bluetooth or something, unless you want to have that white phonecord coming down the side of your neck as if you are a member of the Secret Service.

Now take this idea waaaaay farther. Hmm. ok. Take the headset, and implant it inside the skull. Imagine a cochlear implant, but replace the external microphone with the input from an external source, like our imaginary mobile phone. From the back of the ear where the implant is, extend a microphone boom one to two inches from there towards the front, underneath the ear lobe. The phone will ring literally inside your head, and so you never have to turn off your cell phone in the theatre. Remember our small, strip down base unit? Make it an armband and wear it around your bicep, or heck.. make that implantable too, and place it behind the spine on your back, perhaps somewhere between your shoulder blades.

Man, I can't wait for the day when I can wet-wire an implantable PDA to my brain so I will just *know* the time, where I am via GPS, and remember my schedule off the top of my head.