Excerpt:

the patent remains enforceable for six years after expiration (to collect past damages)

http://www.patentspostgrant.com/lang/en/2009/12/are-patent-claims-treated-like-a-%E2%80%9Cnose-of-wax%E2%80%9D-at-the-u-s-p-t-o

FYI.. a patent is also liable for reexam post expiration.. due to the above.. but we had better be way past that garbage by then...