Findings from jailbreaking a UK o2 activated 1.1.2 iphone, cheers to wrboyce for the support!
My setup: Windows XP, ITunes 7.5
What I used:
ibrickr
http://cre.ations.net/creation/ibrickr
conceited jailbreak package
http://conceitedsoftware.com/iphone/site/112jb.html
o2 edge/visual voicemail fix from here:
http://code.google.com/p/iphone-elite/wiki/o2ukJailbreak
1.1.1 and 1.1.2 iphone firmware restore files (google for them)
1. In itunes connected phone and held shift & clicked restore - selected 1.1.1 firmware file. Flashed and finished with error 1015. Phone displayed connect to itunes picture.
2. Got the phone to boot into 1.1.1 with ibrickr using this method:
* Run iphoneinterface in a Command Prompt window.
* You should see an "r_recovery" prompt.
* Enter "setenv auto-boot true", "saveenv", and "fsboot" (each on their one line).
* Wait for iPhone to reboot -- may take a minute.
3. My SIM was locked and couldn't activate - removed SIM and PIN unlocked it in another phone. Popped it back in.
4. Activated phone in itunes. Phone is now on home screen in 1.1.1

Note: No GSM connection at this stage.
5. Visited jailbreakme.com in safari and attempted to install Installer.app. Didn't work the first time so I had to switch phone off and back on again and retry. Worked this time. Should display progress when its working, before I rebooted it just quit back to home and nothing happened.
6. Installer now on home screen. Installed OktoPrep (in tweaks)
7. Ran ITunes shift-clicked *UPDATE* and choose 1.1.2 firmware. Waited. Itunes activates phone.
8. Running conceited jailbreak - windows.bat. Flashing firmware and installing ssh. Takes a while!
9. Hrmm looks like it didn't work on home screen and no installer. Turning phone off and on. It worked! Now in 1.1.2 and jailbroken

VV and EDGE are broken however.
10. Used putty to ssh into iphone. Ran the following to get o2 edge and visual voicemail working:
rm /var/root/Library/Preferences/com.apple.carrier.plist
cp /System/Library/Frameworks/CoreTelephony.framework/Support/O2_UK.plist /var/root/Library/Preferences/com.apple.carrier.plist
rm -f /var/root/Library/Preferences/SystemConfiguration/preferences.plist
******************
Phone is fully functional in 1.1.2 with visual voicemail confirmed working and GPRS certainly works (no EDGE where I live - damn o2!).
Hope this helps someone out here or gives them the guts to try it too
