Either late tonight, or more likely, tomorrow, is my guess.
They're waiting to see if Arsenal go through in the Champions League, because the two legs of the CL quarter final are either side of the FA Cup semi. If they go through, I would guess that they would play on the Saturday. If they don't, I would expect us to play on the Saturday, as they probably want the two teams with the least travelling to do to play on the Sunday.
I don't think the Blues/Baggies games in London on the Saturday afternoon are as big a deal as some are making out. Given that Man United are playing at Chelsea on Saturday night, I would expect the first semi-final to be on Saturday lunchtime (with the second semi late afternoon on Sunday). So the travel plans of Villa/Baggies/Blues fans wouldn't coincide too much if we were playing on the Saturday.
Saturday lunchtime would be a nightmare, they've always been played late afternoon. Plus the baggies/ blues thing isn't to do with travel, it's to do with them all being in London at the same time and could cause trouble.