Panic Software:

As Mac users, we’ve mostly enjoyed a life free of the worry that has followed Windows users for years. Mac OS X is pretty damn secure. But it could be more secure. As Macs enjoy increased popularity, they become a more attractive target to identity thieves and other criminals. Sooner or later, bad people ruin every nice thing. It’s an immutable law of humanity.So, what to do about this? Code-signing, although it can’t single-handedly fix the problem forever, is a vital weapon in the fight against malware. But many folks are unclear on how it works, or how it helps. Let me try to explain in as close to plain English as I can.

You can read and get more details on Gatekeeper from security expert Rich Mogull on TidBITS and Securosis.