Along the add-on hardware line of thought:

http://www.sparkfun.com/commerce/pro...oducts_id=7917

Dimensions are 1.75x1.75x0.275 --- admittedly, this thing does quite a bit more beyond making and receiving calls.

(This link was bookmarked because one thing I've always wanted to see is a pure-bluetooth cellular module for PDAs and very light calling. No reason to waste all that battery and money on a screen and keyboard and camera and so forth if I can just remote control the damn thing from my perfectly good N800

I imagine the N800's phone tools would be dramatically improved / created before anything else this drastic. Having it function as a cellphone monitor could allow for a number of nifty capabilities --- it's how I implemented push e-mail, for one! I really ought to write that up and post it.

Finally, N800 battery life isn't anywhere near good enough for me to carry it as a phone in any real capacity: that said, it would be nice to be able to stop yelling "IT'S NOT A PHONE"