I planted pepper seedlings from pots into my garden and almost immediately (one day) their leaves looked like this.
I planted about 20 plants and from them about 12 have this problem the others are alright.
Looks like a classic sun scald to me. Seeing as how they're already in the ground, you'll need to find a way to shade them a bit. A little shade cloth or even a cheap section of screening will work just fine.
what Jay said...
+1 mine did the same but but have since grown fine without any negative affects. Luckily mine were in pots when it happened so I moved them to more of a shaded area for awhile.