In order to use the R-Pi as a computer, you need to provide some ‘standard streams’ like a keyboard and an output device (like a monitor). Brad at Liliputing solved this problem by using a Motorola LapDock. He writes:
Motorola’s Lapdock is designed to be a laptop docking station for select Android phones. It’s basically the shell of a thin and light laptop with a keyboard, touchpad, and 1366 x 768 pixel display — but no processor or storage.
In order to connect a Raspberry Pi to a Lapdock, you’ll need a handful of USB and HDMi cables and adapters. You can find a good list of necessary accessories at rpidock.blogspot.com, as well as instructions for connecting the two devices and configuring the Lapdock to receive audio over an HDMI signal.
You’ll also need a USB WiFi adapter if you want to connect to wireless networks, since the Raspberry Pi only includes an Ethernet adapter.
The Raspberry Pi can also be modified to receive power over a USB cable — which means you can use the Lapdock’s battery to power the little computer.
An earlier incarnation of this project by Joe Totten can be found over at rpidock.blogspot.com. These docks look really useful for stuff like this — perhaps you could use it with a BeagleBoard as well, or for that matter anything with USB and HDMI.