If you are an iPhone 3G or iPod Touch 2G user, then you will need to save your SHSH blobs in order to downgrade from iOS 4.1.x (or higher) to iOS 4.x. However you can easily downgrade anytime to iPhone OS 3.x.x from iOS 4.x.x on your iPhone 3G and iPod Touch 2G without SHSH blobs.
iPhone 3GS and iPhone 4 users are always requested to save their SHSH blobs in all cases. For those who don't know: Apple introduced the concept of ECID (Exclusive Chip ID) in iPhone 3GS which is used to generate the SHSH blobs of your device which iTunes check every time you downgrade your firmware from newer to older version of firmware. Whenever a new firmware is released, Apple stops signing the old firmware, hence preventing you from downgrading your iPhone firmware to the older version.
Before iOS 4.0, only iPhone 3GS and iPhone 4 users were required to save their SHSH blobs but now Apple has introduced a new soft SHSH blobs check in iOS 4 which prevents iPhone 3G and iPod Touch 2G from downgrading their firmware to older versions. As this SHSH blobs check is not present in iPhone OS 3.x.x, so you can easily downgrade to iPhone OS 3.x.x from any firmware version without SHSH blobs. These SHSH blobs are useful only when you want to downgrade iPhone 3G & iPT 2G from iOS 4.x.x to iOS 4.x. [via Dev-Team Blog, Cydia image via iH8sn0w]
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