Google Operating Systems spotted Google testing large Google Checkout buttons underneath AdWords ads that are from merchants that accept Google Checkout. Here is a comparison image:
This begs the question, is Google going to far with giving Google Checkout merchants an unfair advantage or not? More discussion at Google Pushing Google Checkout More and New Google Checkout Promo; New Google Trust Worries.
Postscript Barry: Danny wrote an entry that has more on this named Google Checkout Blogs On Recent Promotions.