Fr. Jim Tucker puts the brakes to speculation on women cardinals: The chief problem with lady Cardinals is that the Cardinals are always clergy of the City of Rome, which is why they're given a church in the City and why they're divided into the ranks of Cardinal-Bishops, -Priests, and -Deacons. Their tie to the Pope (and the reason they get to vote for a new one) is in virtue of their being the "hinge" clerics of his diocese.

The so-called "lay Cardinals" of the past were all (to my knowledge) ordained clerics: the post-V2 mind stumbles and calls them laymen because we've forgotten about the minor orders, which (even if they weren't a sacrament) were indeed (and, in the Eastern Churches, still are) ordinations to the clergy. They were also much easier to get out of than were the major orders: so, if you're the scion of a powerful family and are a Cardinal ordained to the lectorate, and then along comes a great marriage proposal that would serve the family... you can just return to the lay state, give up the Cardinal's hat, and get hitched. Charles Borromeo was faced with precisely that dilemma.

Anyway, that clerical character is why Cardinals should always be men. To say nothing of how silly it would look to have women dressed in pontificals, which Cardinals may use even if they aren't bishops, or even birettas.

I do rather like the idea of mitred abbesses, though. And minor orders.

Be sure to visit Fr. Tucker's site. Highly recommended: His recent posts on Jacobites, priestly vestments and life at the North American College.