The new version checks in as Build 8L1, and is for the GSM iPhone 4, iPhone 3GS, all iPads, and the third- and-fourth-generation iPod touch. A separate iOS 4.2.10 (Build 8E600) is available for the CDMA iPhone.

According to Apple this update ”Fixes a security vulnerability with certificate validation.”

This update applies to:
iPhone 4 (GSM model)
iPhone 3GS
iPad 2
iPad
iPod touch (4th generation)
iPod touch (3rd generation)

You know the drill … hook up your iOS device to your computer, fire up iTunes and hit the ‘Check for Update” button.