Im writing this because ive helped about 7 people the past two days doing this and im doing this to save me some time too...
OK if you have bought a iphone and you want it to be jailbroken and your firmware is 1.1.2 just follow my directions here and you will be jailbroke in no time at all....
First thing you want to do is download Firmware 1.1.1 this can be found here..
http://appldnld.apple.com.edgesuite.net/content.info.apple.com/iPhone/061-3883.20070927.In76t/iPhone1,1_1.1.1_3A109a_Restore.ipsw
that is the link for 1.1.1
First thing you want to do is connect your iphone to your cpu and hold the home button and the power button for about 10 seconds when the screen goes black release the power button but continue to hold the home key until you see a message saying that there was an iphone found in dfu mode or restore mode..
After you see that you want to hold shift and click restore and restore phone to the 1.1.1 software that you just downloaded.
At the end of the restore you will get an error messge 1015 this is good.
next you want to close itunes and make sure its closed..
Now you want to download app tapp it can be found here....
http://www.nullriver.com/~adam/AppTappInstaller.exe
now run app tapp make sure you select firmware 1.0.2 in app tapp even though thats not the firmware you have your doing this to bring the phone out of recovery mode...
if you get a message saying to close ituneshelper open itunes an close it again an run this program.. You will get a message saying send to iphone failed this is good....
At this point close itunes and unplug the iphone from the cpu.. On your screen you will see that it says slide for emergency... Connect your iphone back to the computer and open itunes and your phone will now say waiting for activation.. Now you can slide the slider to get to the home screen...When you get to the home screen on your phone the phone wont work at all....
So now turn on the wifi on the phone and go to jailbreakme.com from your iphone... Once your on that site scroll to the bottom and click install for that program on the bottom.. its called appSnapp
Once this is installed you will see installer on your home screen of your iphone.. Click this and go and click all packages and scroll down till you see OKtoprep and click that and install it. A message will pop up saying you are ready to upgrade to 1.1.2.
At this point go back into itunes and click upgrade and upgrade back to 1.1.2 this will take a lil bit for it to finish but just wait..
Once this is finish you will see that installer has disapeared from your iphone home screen dont think that you did something wrong because you havent.
Next you want to go to this site right here
http://conceitedsoftware.com/iphone/1.1.2-jailbreak.zip and download the software right on that screen...
Once that software is installed on your cpu open that file up and click on windows.bat and it will ask you if you want to run it or exctract all.. You want to extract all...
Right Now close itunes all up and Open that file you just extracted and go to the windows.bat file and click it again this time click run... A black command box will pop up and another box will pop up on this box it will ask if you if you want to reboot or jailbreak on the bottom of it and just click the jailbreak button...
Now the software will run it will take about 5-8 mins or so to do its thing durning this time your iphone will reboot a couple times and when you see that your iphone has came back on Your all done you will see that the black command box says rebooting iphone and it will be done if your iphone is just sitting on now and not rebooting... Now you can close that black command box and your all done..
You will Now see the installer icon on your home screen again....
Congrats your done....
have fun customizing your iphone if you need any help feel free to ask me but try the above steps first thanks