Upgrading to 1.1.2 from 1.1.1 is EXTREMELY easy.
Go into Installer and install OktoPrep from the Tweaks (1.1.1) menu.
Then, UPDATE to 1.1.2. Do NOT shift-click or even click on the Restore button. Update only.
After that's done, go to
http://conceitedsoftware.com/iphone/site/112jb.html and download the zip. If you're on a mac, run jailbreak.jar from the command line. If you're in Windows, run windows.bat. Make sure you have the Java Runtime Environment installed in Windows if it fails when you run it.
Follow the prompts on screen, and voila. Jailbroken.
As for upgrading from 1.0.2. Do a shift-click restore to 1.1.1, then follow the steps above after you jailbreak 1.1.1 at
http://jailbreakme.com
Jailbreaking a OTB 1.1.2 is a little more tricky, but it's still very easy. Just involves the use of either Indepenence in Mac or iPhuc in Windows to get your phone out of recovery mode after the 1.1.1 downgrade.