How Small Could/Should It Be?

Since it seems I'm an idea person more than an implementation person - let me throw this out for the world to use (but I wouldn't turn down a consulting fee). How small should a cell phone be? Imagine if you will a cell phone with no keypad and no display screen. The limiting factors would then be the battery, antenna and speaker/mic. So picture something the size of a bluetooth headset with one button - that's it!

Now how would you interact with it? I imagine a bluetooth connection to a computer would allow easy access for setting up a contact list or appointment calendar, but the main interface would be through voice recognition. Many advanced phones already have speaker independent voice recognition, so you could say "Call Home" or "Dial 5 5 5 4 5 6 7" but why not extend the functionality? How about accessing a stored calendar by saying "List Appointments" or "Set Alarm for today 3PM"?

OK, next step beyond, have music stored on the device - but wait, that means we're probably talking about increased size again. What if there were audio only online content? When you're not on a call you could say "Stock Ticker" and have your favorite stock quotes. Say "Headlines" and have current events playing in your ear. Or how about this, online radio stations or music collections? Can you see a bunch of people saying "Next Next Next Play"

The growing number of people plugging in during commutes is ever evident by the headphones. In Boston they have cell repeaters in the subway tunnels so you can actually keep a signal underground although the noise level may cause problems with a voice recognition system. Anyway, just a chance to get an idea out there in black and white.