Getting your "keys signed"?

[longshadows]

I have read several posts, warning jail broken iPhone users not to upgrade to firmware 3.1.3. Okay, no problem, but the notices go on to state...

"Make sure you *rush* into Cydia or Rock and get your keys signed on the front page if you have not. This will make certain you can always restore 3.1.2 even in the future using Saurik?s server."​

Could someone please clarify what this means? I don't have a clue about how to do this.

Thank you very much!