This is on the FAQ on ATP site:
Q. What is the ranking structure and formula in 2012?
A. In November 2011, the ATP Board passed a rule to allow more flexibility in which tournaments count toward the ranking of a commitment player in 2012.
Commitment players (top 30 in 2011 year-end ranking) can count their best six (6) results from ATP World Tour 500, ATP World Tour 250 and other events (Challengers, Futures, Davis Cup, Olympics) toward their ranking. To count their best six (6), players must have fulfilled their commitment to 500 events - 4 total per year (at least 1 after the US Open).
So is this structure a permanent change or just a change for 2012 to accommodate the Olympics? Anyone knows?
I think it's permanent but your question is valid as the Olympics may well have been the main reason for that change (although not the only one : probably players were annoyed by the complex previous rule) : it's better checking at the new rulebook (not having access to the ATP website at my work, I can't check it at the moment)