Hi, I have one of these keyboards also (the keyboard that comes with the mx duo). Lineak, or whatever special program you use, works for most of the keys, but not all. Let me guess, you can't use the webcam, messenger/sms, shopping and itouch keys also.
The problem is that 2.6 kernels can't see keycodes over 240 properly. Try using the command showkey to see the codes produced by the keys in question. The f-keys produce keycodes 336-347 for me.
To get the keys working though, they also have to send a scancode. You can see these with showkey -s. Unfortunately, at least for me, the special f-keys don't have scancodes.
There's a thread over at LinuxQuestions where a few people have the same problems:
http://www.linuxquestions.org/questions/history/125333
Also, there's a thread on the keyboard over at the lkml:
http://lkml.org/lkml/2004/4/25/76
Of course, that was all assuming you have the same keyboard. Apparently it "just works" in 2.4 kernels, but I haven't verified. I would try it out, but my root partition is reiser4 and AFAIK reiser4 doesn't work in 2.4.