It appears that the Google Checkout icon feature was flipped on for all ads earlier today. The first report of the bug was issued at WebmasterWorld at 7:55 AM (EST), and then more reports starting trickling in. Now the bug seems to be patched, but it took about an hour and thirty minutes to fix it.
SearchViews captured a screen shot of their reprise media sponsored listings sporting a little blue shopping cart icon.
Forum discussion at WebmasterWorld and Search Engine Watch Forums.