Если вы прользуетесь разного рода турбосимками и проксисимками, то ни в коем случае не обновляйте прошивку до 2.2.

Дело в том, что вместе с апгрейдом софта вы получите и апгрейд т.н. baseland, которая включает и аппаратную часть (модем), вследствие чего ваш телефон перестанет работать со всеми gsm сервисами. Апгрейд baseland перекрывает все известные дыры, которые использовались турбосимками и также перекрывает возможность сделать downgrade модемной части. 

Т.е. даже если вы откатитесь до прошивки 2.1, ваш baseland останется версии 2.28. А это пц телефонной части.

Вот такие грустные дела.

iTunes (or any external program) can't directly program the baseband NOR. The best it can do is to ask the current baseband, "please replace yourself with this version I've just uploaded to you". Then it waits for a reply -- either the current baseband verifies and accepts the upload, or it says "no thanks, go away".

It turns out that part of the verification the current baseband does is a version compare. If the version is the same or less than the current version, it politely declines the programming request. It also declines (though maybe not so politely) if you try to trick it into programming a block of code that fails the checksum and certificate check.

Baseband 101
The ‘baseband’ is the generic name given to the internal components of the iPhone that handle the phone calls and Internet access. This ‘baseband’ is a tiny and unique independent computer system that runs inside your iPhone, it is separate to the main system that handles the applications (such as email and google maps) and it talks to the main part of the phone over an internal communications network. Think of it like a cable modem or other peripheral that is attached to your home PC that needs occasional updates. When a software update is released and presented to you within iTunes the baseband is sometimes updated (to fix bugs or add new features). The 2.2 update for the iPhone 3G contains such an update, so running the vanilla updater straight away with iTunes will reprogram and update the baseband. This could be bad for certain people, depending on your ultimate aim.