I have started on a tutorial series, which might be interesting. I am now at post 1. So not so much yet, but will go through I/O kit, hopefully in a more informal style than found on apples dev pages.

http://www.superhai.com/blog/

I have not any timeframe, but will try to post a new part each week.

Hasn't Apple allready got a quite perfect IOKit + kernel extension tutorial in pdf form?
It's the one I read a while ago and found it to be quite informative and easy...
Maybe it's just me because I used to be a Linux coder...