Pocketop(http://pocketop.net/) ships a foldable keyboard with a wireless interface (IrDA/InfraRed) which is compatible to Palm PDAs, various Pocket PCs and the Sharp Zaurus Linux PDAs. The keyboard was tested on a Zaurus SL-5500G and a SL-C750.
The driver for the Zaurus is not available on the shipped CD. You can get the driver from Pocketop, but you have to register there first. At the moment, that driver has not yet arrived here.
Fortunately, you can also download the IRK package from the Craig(http://angela1.data-uncertain.co.uk/~zaurus/). The applet hooks into the menu where you can select the various input methods (virtual keyboard, unicode, etc.).
After setting IRK up to use the Pocketop keyboard and the right Zaurus, the normal keys + "Punct", "Num", "Alt" and "Shift" work. The Function key does not work correctly, though. When you hit that key, you are thrown to t= he "Applications" tab and subsequent hits cycle through the tabs at the top of the screen. So you lose the "PgUp", "Home", "PgDown", "End", "OK", "Off", "Esc". Perhaps this can be configured somewhere in the keymappings of the settings dialog.
Inconveniently, the 5500 has its IrDA port on the left side, so you have to turn your head (or your look) 90 degrees if you want to read what you are typing. One solution to that would be to construct a mirror system which leads the rays from the Zaurus to the keyboard. The keyboard already ships with a stand which also features a little mirror, but that mirror is positioned at the top of the Zaurus when it is put into the stand, so that it is pretty useless. But there is also "Rotation" plugin available for QTopia which you can download from the Zaurus Software Archive(http://killefiz.de/zaurus/showdetail.php?app=1080) at Killefiz. Alas, it does not seem to work with Sharp-ROM 3.13, although the two rotated fonts have been installed. When you choose "Rotate" from the menu, QTopia is restarted but reappears in the same format as before.
The situation is better here: You can rotate the display so that you can type and read as usual.
See here for other keyboards working with Linux PDAs.