Look at the reviews for this card:
http://www.newegg.com/Product/ProductReview.aspx?Item=N82E16839314004
One reviewer says that FreeBSD 7.2 recognized it without issue. If that's the case, it is likely to work with pfSense, but I'm not sure which driver it would use. One review says Marvell Yukon, so probably msk(4).
From what I can tell, it's possible that there isn't really a distinction with ExpressCard from PCI-E cards, except that I wouldn't try to hotswap them. :-)