One person has come very close to guessing what's happened (but I refuse to say who it is). Yvonne summed it up nicely (and practically quoted a later part in the story) - he was absolutely fine and super inside the truck, it was only when he went outside that things went south for him. But it's not virtual reality.
Sammie is right, too. It's not another color of Kryptonite either.
I'm still tweaking some sections of four, but I promise I'll post before I leave this weekend.
Thanks guys! I'm glad I've got you thinking.
ETA:
Early on, I was betting Lois put Clark in the hospital after she found out CK=S. Now that completely cracked me up. The bonus is that now he's vulnerable, she could totally beat him up. <g>