I now have an i-Tech Bluetooth Virtual Keyboard (stupidly pulled the link from the sources - the site is otherwise not sensibly usable and without JavaScript there are no direct product links - clear case of "stupid designer syndrome"). Very nice - the keyboard is projected onto the table and then scanned using an infrared barrier above it. The virtual keys work excellently.
I bought it from Expansys, but they sent me a UK version - the power supply doesn't fit German sockets. The layout is of course always English, but I would like to have the power adapter in the appropriate version (that's why the strange links to adapters from English to Schuko).
Integration with the Nokia 770 is very simple - you just get the Bluetooth Keyboard Plugin and install it. Warning: if the bar with the icons at the top of the display is already full, the Bluetooth icon may no longer be displayed. Then you have to part with one of your status bar plugins.
After that, you just have to go to the plugin settings and, with the keyboard turned on (and reset for safety), select it from the list of available devices and pair it. After that, the plugin automatically recognizes the keyboard when it is turned on - you don't have to connect manually, just restart the tablet, turn on the keyboard and off you go. Turning off the keyboard is also properly recognized by the tablet as a disconnect.
Oh, and this laser-projected keyboard is not only extremely practical, but also absolutely cool.