A few years back Palm came up with a companion for their Treo Smart Phones that was nothing more than a larger screen and larger keyboard that the smartphone attatched to for easier reading and text entry.

see:
http://en.wikipedia.org/wiki/Palm_Foleo

Of course, it failed!!! They forgot to test the idea with the users of the Palm Treos and discovered that for $500 they were happy to continue to use the smaller input keyboard and smaller screen on the phone! Surprise, Surprise! What did sell to Palm users, myself included, was a great piece of software that moved the Screen and function of the Palm Treo to your computer screen called PDAreach made by Junefabrics.com. And as it was software you didn't have the issue of one more piece of equipment to pack and carry when you went somewhere.
It seems more practical to move your mobile device's screen to your desktop or laptop in a window that you can resize that to have a piece of hardware dedicated to a job that could be handled by software. Then you plug your BT headset, connect to the laptop of PC with USB (or if you want to get really fancy by Wifi - probably overkill since the purpose is to simply "move" your screen) and you now have the same function with the same hardware you already have and you can use more powerful software for the real tasks you do everyday, like Excel spreadsheets, Access database, etc.

In retrospect, the lesson is that no matter how "cool" that device is, you should listen to your customers and maybe use outside the company users before you get to the point of product in package. Palm found out the hard way that in the end the customer will be the final judge, and you can generally get that feedback a lot quicker if you listen. Palm quit listening at some point in time (there were no links on their website for suggestions or comments) and eventually their customer base went away.

Don't need another piece of equipment, my laptop is small, powerful and light. Move the screen, leave the extra equipment in the box!