Save your shsh key!!
Every iPhone has a unique key that is required when installing a firmware on the iPhone. If you do not save your shsh key Apple will dictate what you can and cannot do with your iPhone.
Important Information read carefully
The following information applies to all iPhone 3G, iPhone 3GS and iPhone 4 models.
Apple only supplies the key for the latest firmware so if you do not save the key you will not have any choice but to do what Apple dictates as Apple's server signs firmware SHSH requests made from iTunes during the 'Verifying firmware…' phase of a restore. This can pose major problems for anyone who wants to jailbreak or unlock their iPhone.
If you are trying to restore a firmware that Apple is no longer signing, you will receive a 30XX error from iTunes and you will not be able to restore that firmware version to your device. /Period/. The ONLY way you can restore that firmware is to point to Cydia via your hosts file OR run TinyUmbrella and use the SHSH files you have saved /previously/. I cannot emphasize this enough.
ONCE APPLE HAS STOPPED SIGNING FIRMWARE SHSH REQUESTS FOR A PARTICULAR FIRMWARE VERSION THERE IS NO WAY TO GET YOUR SHSH FOR THAT PARTICULAR VERSION.
As an example, Apple stopped signing firmware 3.1.2 SHSH requests WAY back on Feb 04, 2010. What this means is that if you did not save your SHSH BEFORE Feb 04, 2010 YOU cannot restore your device to 3.1.2 no matter what.
This is why it is so important to save your SHSH files.
As soon as you upgrade your iPhone to a new firmware you MUST complete the following steps to protect your future iPhone jailbreak.
Without it you are at serious risk of locking your iPhone. Follow the steps below to save your shsh key and protect your future jailbreaks.