I hear a lot of discussion on "once you jailbreak 3GS, it can always be jailbroken untethered. Can someone elaborate on this for me please? I just jailbroke a friend's 3GS by going to jailbreakme.com but as such, she's obviously on an older firmware. If she wants to update her firmware, what must I do to insure she can remain untethered?
There are 2 types of 3GS phones, old and new bootrom. Old bootrom phones are succeptable to 24kpwn, therefore they are always untethered when jailbroken by current tools. New bootrom phones are not succeptable to 24kpwn, therefore they require an additional untether exploit to be jailbroken and untethered. If the phone is a new bootrom, you cannot do anything to it to insure it is always untethered. Like everyone else, you are dependent upon an untether exploit to make the jailbreak work untethered for your device.
Tutorial for updating iPhone 3GS whilst preserving baseband for unlocking:
There are two bootrom versions of the iPhone 3GS. They are old (359.3) and new (359.3.2). It is best to confirm by actually checking the bootrom version however the Serial Number or Manufacture Date of the board is a good indicator. All boards manufactured prior to the 40th week of 2009 are old bootrom. From week 40-45 is a cusp period where either version may have been used. Also remanufactured iPhones indicated with a serial beginning with 5K or QR may have recycled an old bootrom chip.
Old bootrom will always be untethered jailbreak based upon the exploit used. Old bootrom also does not require the SHSH blobs to restore to any given iOS version.
New bootrom requires an untethered jailbreak to be released for the given iOS version. New bootrom requires SHSH blobs to restore to any version of iOS that is not currently signed by Apple.
Here is a general guide for updating or restoring a new bootrom 3GS outside of the signing period:
You can grab what you need from here: JailbreakQA Links
Please post in comments if you have further questions regarding the procedure for your situation. For most accurate advice, please access the Settings App on your device and provide from General>About the Model, Version, Carrier, Serial (first five digits), and Modem Firmware. This info can also be accessed using Tiny Umbrella.
Your info will look something like this:
Model: MC131B Carrier: O2 UK Version: 4.3.3 Serial: 88935xxxxxx Modem Firmware: 5.11.07
Hope this helps, best wishes and good luck!