The characteristics of female-friendly games include: easy to learn but hard to excel at; easy to pick up (i.e. can be played interstitially); mentally rewarding. Casual games fit all these criteria, and non-casual games are fulfilling more and more of them (for example, Animal Crossing Wild World and World of Warcraft).
This is something of a generalisation--you'll find female gamers playing any game, on any platform--but it's useful to get this message across to game designers. However, similar lists of criteria for "female-friendly" gaming have surfaced after Women in Games events for at least the last year--there seems to be little novelty left in this extremely specific niche.
[Via Clickable Culture]