From what I've read of these forums, so far, you cannot unlock the 1.1.2 iPhone as of yet, and most of us with out of the box 1.1.2s are desperately waiting for a way to use our sim cards on it.
However, downgrading to 1.1.1 is possible, I know because I've done it with an out of the box 1.1.2 I bought yesterday.
Basically, the guide didn't work for me, but here's what I did:
1. Boot in restore mode (1.1.2 restore mode has a picture of a cable going into an itunes logo with a disc).
2. shift-click restore in itunes and select the 1.1.1 ipsw file that's everywhere.
3. wait until it finishes, and itunes will give you an error.
4. ***This is where the guide stopped working for me. It said to use iphuc32, but everytime I entered a command the thing died. Someone in the thread said they used iBrickr, and so I thought I would give it a shot. While still in restore mode, right after given the error in itunes, press okay. Then go to ibrickr, and press boot my iphone. If everything is working, you should see your background turn red, and your iphone should boot into 1.1.1.
5. From there, use the guide's method of getting to safari and going to jailbreakme.com, etc.
This is what's worked for me, but it seems that different people are having different results, so who knows.
Though, no SIM cads other than AT&T can be used for now.
Good luck.