Great find. Very interesting, but not too surprising. The detail page (when you're reading a book excerpt) is interesting -- you lose the Google logo and design 'shell' and get an all-text page.
Looks like every detail page has links to buy the book at Amazon, B&N, or Books-A-Million. There are some sponsored links down at the bottom of the detail page, but not on all searches. There's a "Google Print FAQ" link at the bottom which states:
On Google Print pages, we provide links to some popular book sellers that may offer the full versions of these publications for sale. Book seller links are not paid for by those sites, nor does Google benefit if you make a purchase from one of these retailers. In addition, these pages show contextually-targeted AdWords ads that are served through the Google AdSense program. During the initial phase of this beta test, advertisers will not be charged for clicks they receive on these pages, and neither Google nor the publishers with whom we work profit from these ads.
And the print.google pages aren't quite <ahem>optimized</ahem> as yet. For titles I confirmed are in the print.google index I found it almost impossible to come up with a SERP that includes a print.google result. Tried about 10 and only one popped up in position 20, and that after putting the title in quotes.
Unless it's going to markedly change over time best bet might be to include it as a feature, such as Froogle, or another tab (they still have a bit of room).