Of course, there is obviously no way to know for sure, and people who watch Google closely can attest to the fact that these indications sometimes don't mean anything -- Google Calendar has had a number of these "Google Blips" which tend to create buzz. This information is very interesting but should be taken as "speculation" because that's what it is.
Google's terms of service for their video upload program changed on Friday morning, more specifically the wording now states that they might allow publishers to "rent" their videos. This creates more of a need for something like Google Purchases which would allow renters to pay owners. So I started watching Google Purchases (code name "sierra") a little closer.
In the very recent past, users were greeted with a login screen when visiting purchases.google.com, even if they were already logged in. Any attempts to gain access would always take them back to this same screen. Now when users log in, it actually takes them somewhere -- to the Google Account Settings page -- but there is no link for Purchases (yet). This could indicate that they are a little closer to launching the service, or that they are just messing with people who watch for changes. Both are reasonable explanations.
To thicken the plot, here is another "Google Blip" that could relate to the launch of this service. In the past I noticed that Purchases could have a downloadable component. The address dl.google.com/purchases/wallet-setup.exe (or any other file name) now redirects you to your account settings page too -- rather than the previous 404 (which means that the file does not exist).
A related downloadable component could be used for any number of things. It may facilitate micro-payments, let you store information like credit card numbers locally, or be a plugin for Gtalk that lets you "pay" people just like you would "call" them. It could also be the software that lets you "rent" videos just as the google video upload terms of service now imply. I don't know what this software will do, but this evidence suggests that it will exist.
These two pieces of information tend to suggest, at minimum, that something is happening with Google Purchases. With the upcoming services that could accept payments for things -- like "renting/buying" videos -- it is likely Purchases is getting closer to launch each day.