This is a tedious argument.
Personally, I don't think there will be a Big Bang in April. I think what will happen is that those people who say "aww nothing's going to happen, you're scaremongering us" will crow about it for a while and wallow in self-congratulation either that they've dodged a bullet or stuck it to the IT man.
However, slowly but surely there'll be the odd user here and there who'll find their XP machine doesn't work quite right. They'll call their IT guy and get one of two responses: either "XP? Are you mad?", or an IT kid who spends an inordinate amount of time trying to fix the issue only to either have it repeat the very next day or have to admit defeat.
The end user, not knowing their arse from their elbow, will chastise the IT guy for not making it work, and will dismiss the counter-argument of XP being inherently flawed from that moment on as unproven.
It'll therefore be a slow-moving mass of pain to a greater or lesser extent depending upon how much reliance is placed on the system by the end-user, how we'll they've achieved their own updating obligations, and how soon an API exploit becomes generally available.
All in all, it's going to be miserable for all concerned, but to what extent we will have to wait and see.