A WebmasterWorld thread asks what does the "google_ui_features = "rc:0";" line represent in one's Google AdSense script code?
It actually stumped me, but the response made me go "doh!"
It represents the relatively new AdSense rounded corners feature.
So now your ads can have round corners, and they will be defined by the rc:#.
Here is an example of an ad with an rc:10 value, rounded corners:
Here is an rc:6 example, i.e. slightly rounded corners:
And here is a rc:0 example, i.e. square corners:
Forum discussion at WebmasterWorld.