It was, I played the demo for it, but because of the different publisher it isn't on Steam.
PotC wasn't released for the PC, for whatever reason (Disney?).
I'm on a bit of a LEGO game spree this christmas. I bought PC versions of:
Lego Batman 2 - £12 - Amazon
Lego Lord of The Rings - £17 - Amazon
Lego Harry Potter 1 and 2 - £9 for both on Steam.
Need to find Lego Pirates of the Caribbean for PC next
Batman 2 and LotR are both really amazing though. Batman especially feels like such a drastic leap above the past games.