it will be the same as always, the attendance will read 90,000 but there will be about 15-20k empty seats. Not sure it technically counts as a sell-out when a quarter of the stadium is already sold to people who never turn up, but it will read as that.
Another thing to consider is the amount of FA Cups chelsea have won recently, if they get through, some might not be arsed to go if wigan are the opponents. I know that wouldn't be our mentality, but lets face it, a lot of their fanbase of jumped on the bandwagon, they probably don't view the FA Cup as all that.