According to this CVG piece, Microsoft's said that free Live play for certain 360-PS cross-platform games is only for a limited time.
"Last week we announced that we were taking off the Gold-only restriction for multiplayer gaming over Games for Windows- Live," said a Microsoft spokesperson.
"For a limited time, we are offering free online multiplayer on Xbox 360 Games for Windows titles that offer Live cross platform play. This promotion won't last, so we encourage everyone to take advantage of it while they can."
Play on certain games was dropped after Microsoft decided to stop charging for Games for Windows Live.
More through the link.