I've heard and read conflicting things as to whether they are. I'm not 'expecting' us to get there but it'd be a nice extra incentive to win the next game, not that one should be needed!
Yes, read it when Wembley was being built They need to pay for the place rather than preserve the tradition of the cup
See the link below, semis are played at Wembley 5th/6th April: http://www.wembleystadium.com/events/majorevents/