It's entirely possible that Google will give, upon request, the page scans for any public domain books they've scanned to established groups like Distributed Proofreaders for conversion into proofed SDT, so long as Google gets a copy of the resulting high-quality SDT.
My guess is that part of the deal is that the libraries are going to get copies of those page scans, and they will probably make them available in various ways in addition to whatever Google does with them. By the way, it's astonishing to me how far OCR has come in the last 10 years. I think the low cost of storage has made page image storage of many historical documents feasible, relatively suddenly, and that means that the problem of OCR'ing handwritten text, odd fonts, early books, and other similar things has suddenly become a hot research topic. Bill