According to the application, "P.S. Thanks" is meant to be an "incentive rewards program geared toward quantifying, identifying and motivating top consumer base with the goal of promotion and sale of video game hardware and software." In short, Sony wants to create a PlayStation loyalty program.
IGN hypothesizes a few possibilities. The service could be a program akin to Club Nintendo that awards frequent buyers with rewards. (Most recently, top-tier Club Nintendo members were awarded an exclusive downloadable game, calendar, and Mario hat.) "P.S. Thanks" could also refer to a Trophy reward program, one that has been rumored since the inception of Trophies in the PS3 firmware. Certainly, that's something a-certain-someone would love to see. Update: SCEA offers a comment: "As you know, we have a long history of creating unique loyalty programs such as PlayStation Underground which started with the original PlayStation and has evolved into the Gamer Advisory Panel (GAP). We continue to look for ways to expand this program and simply are evaluating new naming conventions for this evolution."
[Thanks, Jeff B.]