I've had the same problem with Shebeest shorts, though I'm not sure it's the fiber as much as the fact that that seam is awfully abrasive. Anyway, same thing happens--10 miles or so down the road and I'm ready to ride home shorts-less.
Shebeests seem pretty popular, so maybe it's something about the shape of me. Anyhow, I've found an alternative that doesn't rub me raw--Terry T-Shorts. I just got my first pair about a month ago and I've worn them in my early training for a week-long bike tour this summer. Not the longest rides in the world, but in the 20-30 mile range, and the Terry shorts haven't let me down yet. Same goes for their new 2002 knickers.
I've heard some women say Terry shorts don't exactly wear like iron, so I don't know how long they'll last. But they fit beautifully and they feel great, so I'm treating them right. Not cheap, but what bike shorts are?
--J
"Do I contradict myself? Very well, then, I contradict myself." --Walt Whitman