Those of you with jailbroken iPhone 3G and iPod Touch 2G devices may now have noticed Cydia starting to save your SHSH blobs, just like it does for iPhone 3GS, iPod Touch 3G, and later devices. That’s because starting with 4.0, Apple started putting a “soft” SHSH blob check in the firmware. The SHSH blob check is very real in the sense that if iTunes can’t get your blobs (because the Apple signing window has closed), the iTunes restore will error out. But it’s “soft” in the sense that those devices can always use redsn0w or PwnageTool to get past the error (the bootroms themselves for those devices don’t require blobs to be in the firmware files, unlike the newer bootroms).
Furthermore, since the 3.x IPSWs for these devices don’t enforce it, you can always restore to 3.x IPSWs outside of any signing windows.
So, Cydia is doing this to allow you to continue to use iTunes to restore to 4.x on iPhone 3G and iPod Touch 2G outside of Apple’s signing window without needing to use redsn0w or PwnageTool to get around Apple’s annoying new restriction.