I know it's a sign that a speaker has been running for a while at high levels, and the voice coils are becoming warm...
But how bad is it really? Last night before the musical at my school I had a nice crowd around my car showing off my system, probably for about 25 mins. or so at RMS levels. After which the dust caps on my Assassins were a bit warm, not hot, just warm a bit. They still sound perfectly fine at high levels (after they had cooled for several hours), and there is no detectable mechanical failures either.
I was just wondering how bad it is if they get warm once in a while...
I think the major culprit was when I started playing Speakerboxxx Intro. Until then it was just What You Know and Throw It Up. Still though, at high levels it was pretty impressive.
People inside the choir room (about 80-90 yards away from where I was parked) could hear my system, and all the windows were closed, and the school is completely brick. So I'd say for being sealed that's pretty decent lol.