Google has opened up an interest form that you can access over here that lets you try to get included in the Purchases on Google beta.
The buy now button was demoed a couple years ago and looked like this:
Anthony Capetola first noticed this and covered it. The option to fill out the beta form is in the Merchant Center Programs section in Google Merchant Center. Here is a screen shot:
Here is an example of what it looks like, at least in the search results as Google has been testing it:
So if you are interested in this beta, fill out that form.
Forum discussion at Twitter.