Picked up one of these to go with the Playstation 3 and it after pairing it works straight off. Next challenge was to get it working with Linux so that I could use it after I installed Gentoo to the PS3.
I enabled the following options for Bluetooth in the kernel (tuxonice-2.6.28)
Networking --->
Bluetooth subsystem support --->
<*> L2CAP protocol support
<*> HIDP protocol support
Recompile install and reboot into your new kernel.
I opted for Blueman as had used it succesfully to connect to my N95. I'm using the SVN version under release 0.6 as >1.02 is currently hard-masked in Gentoo and unmasking it and the associated dependencies created a world of hassle.
The keyboard showed up after a quick list of bluetooth devices from Blueman so the next step was to bond and connect to it. After a few false starts this proved relatively simple. Click on the Bond button and a dialog will appear, enter in any keysequence you want but make sure you know what it is (something simple like 123456 is straight forward and easy to remember). The click on 'OK', then go to the Keysonic 540BT and type in exactly the same key sequence, and hit Return. You're keyboard should now be bonded.
Connecting seems to be a bit flaky but is stable when done from the Edit > Services > Input > Settings > Connect menus/dialog. Unfortunately I don't yet get any output from the keyboard, no matter what keys I press, but I've posted a request for help on the Blueman forums and will hopefully have that resolved (and will update this page when resolved).