Apparently, they may. Especially IEMs. You guys should have a look at this publication (
http://www.aes.org/e-lib/browse.cfm?elib=15786). Basically it states that when sound waves get trapped in sealed ear canal, air pressure builds up inside and can boost the signal even up to 50dB! This leads to ear drum excursions up to 1000 times greater than in normal listening conditions. That's pretty scary to say the least. To make things worse, this build up triggers Stapedius reflex, which makes you perceive the loudness much lower than it actually is. This publication is pretty new and I really hope that more proper studies will be made in the near future regarding IEM safety, because now it looks like we still know very little.