AppleInsider is reporting that Snow Leopard will include an "anti-malware" feature. Here's a brief excerpt from their story:
"In a screenshot that has been widely circulated, originating from the intego Mac Security Blog, Snow Leopard is shown to recommend that a .DMG file be moved to the trash. The OS claims that the file includes 'OSX.RSPlug.A' malware."
Following a link from AppleInsider to another site and then to another, the root of their report seems to be this Intego blog post.