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A new version of TinyUmbrella, the tool that allows you to take backups of SHSH blobs (ECID SHSH) has just been released to support iOS 4.3.1. TinyUmbrella also protects iPhone 4 from baseband updates.
Apple released iOS 4.3.1 yesterday, which includes improvements and bug fixes.
The backed up SHSH blobs (ECID SHSH) are required if you want to downgrade to previous iOS version as Apple usually stops signing them after a new iOS software update is released.