I just had a similar experience with my Thermarest. I bought a Prolite Plus Small last fall and used it for ~200 days of touring, during which I used a sleeping quilt so only a cotton liner separated me from the pad. A bubble (delamination) formed on the upper surface near where I rest my head. I sent it in for a warranty replacement and received a new pad in two weeks. Enclosed was a letter stating that I'd let my pad get too dirty/sweaty/oily and these contaminants wrecked the surface/foam bonding, so the damage wasn't covered by warranty. Despite this, Cascade Designs was still willing to replace the pad.
So now I have a new pad, respect for Cascade Designs/Thermarest, and am more aware of just how clean I need to keep my pad. While I can't avoid being dirty, sweaty, and covered in sunscreen while on tour, and I'm not willing to use a full sleeping bag, I will certainly wash my pad more than every few months. One more thing to bring into the gas station washrooms I suppose.