I had a chat with a guy who had taught legal defense strategy, and he said that he had a hard time coming up with any plausible defense story. He said, "How about a case of mistaken identity--he called an escort service for some kinky scenario, but the maid showed at the wrong time and he made an honest (ahem) mistake.?"
Question then: how could a call girl open the always locked door of the hotel suite? With what card?
If this defense is tried, DSK would have to bring a specific call girl as a witness, and that person would have to testify that DSK met her and gave her one of spare keys to his suite. This would be a strange arrangement - it is much easier for DSK to just open the door when she comes.
The millions that his current wife is planning to spend on his defense can buy a lot of witnesses who will gladly lie on the stand. One little problem might be if DSK didn't ask for extra key cards. It's not like a clerk in a five-star hotel will blithely program a handful of cards to a $3,000 suite. A clerk will ask, and he will make as many cards as you want - but each swipe of the card will be recorded; keys are important things, and it pays to know how many are out there; and it's easy to do since the computer does the programming anyway. It may also be that each key card is unique, and though the lock opens when you swipe any of them the computer knows which card was used; that's how I would program the system.
Another little problem with buying the witness would be in fact that the lawyer would have to abuse his privilege to arrange a consistent testimony. How many high-flying US lawyers will want to risk all they have and will have (for them it's more than for me :-) just for a mere million or two? They are going to get that million just as matter of course anyway, as their fees. Imagine the blackmail power that such a witness, or anyone who can invalidate the claim (or just says that he can invalidate the claim) then would have over the lawyer!
Besides, the trial will also reveal why the senior maid thought that the room is vacant and a maid can be sent to clean it. I'm sure they don't pass notes on napkins in a 30-storey hotel, they use a computer. So how could a record in the computer be made that the guest left? Such a record won't normally be there until you check out and your key cards are voided. One way to do it is to mechanically unlock the door (or block it from closing) then go and check out, then quickly return and enter the suite, lock the door from inside (what for?) and wait. One catch with this scenario is that the elevator may not deliver you to the floor where your $3,000 suite is unless your key card is valid...
I think I read about a UK case a while back where four gang bangers walked (it went to the House of Lords) because her husband had assured them she wanted to realize a long-standing rape fantasy and they should ignore her protests.