I am interested too in dual booting maemo with Kali Linux, but I don't know if I have the knowledge to do this :P

For the xz file, after a little search, I found out this:
unxz file.xz

and I am doing it right now and it seems to work...(but it is really slow!)

Edit: I tried it on my pc(Kali Linux) but it didn't work:
unxz: kali-linux-1.0.5-armel.img: Write error: File too large