With the modification I have sent I can register a HTC Touch Pro to the BS11 and the nanoBTS 1800. If GPRS is set in SYSTEM INFORMATION 3, the phone will not register.
The process is a bit unstable, if the phone is powered on, the location update works as expected, if I try to register manually, it sometime fails without actually communication with the BTS. I don't know the reason for this behaviour yet.
I have good news now, I'm able to register to our bts manually. What I did is download the latest openbsc sources and compiled the whole project, that's it. I downloaded the project using git via port 80, I posted a mail about that. So I guess the GPRS in the Rest Octets have nothing to do, just the SIs were not complete. I'll analyze what went wrong with the SIs.
Oooh I forgot to mention that I use HTC Artemis (3030 I guess).