I have an interesting topic for discussion: I noticed years ago, with my '91 D1, that when I played the 8th fret "C" on the high E string, the sustain was considerably less than surrounding notes. My '01 D2H has the same tendency, too, although to a lesser degree. In discussing this with George, owner of Maple Street Guitars in Atlanta, he suggested that the reason might be because of symathetic resonance from other open strings that have stronger overtones that "oppose" (not his words) the C sustain. That's probably right, since it is less noticeable when I note other strings (but don't pluck)with C scale-related notes, or either totally dampen them (which doesn't make it go away completely, since there is still neck resonance). I have found this in other brands of guitars, too (mostly dreadnoughts), but usually the better ones. Cheaper ones don't have enough sustain and resonance to make this very evident, I guess.
My question is this: if this is true, why the C note, and no other ones? Every note has other non-harmonic sympathetic notes that occur alongside. Is it just a question of volume and my personal guitars (don't think so, since I check every guitar I play now and it is RARELY on any other note)?
This oughta stir up some of you tone- and gear-heads...

If the other strings are not permitted to resonate by damping, I don't see how the sympathetic resonance theory can hold up. Try tuning your high E down a half/whole step (or up a half/whole step) to see if the note on the eighth string still sustains less. If it does, doesn't that suggest that the open strings' sympathetic vibrations (or neck resonance, a term with which I'm unfamiliar) are less suspect?

bishopdm:
I lowered the string 3/4 to a whole tone. The short sustain problem went away, at least 90%, lending credence to the sympathetic string theory. However, it could still be an overly active neck resonance (by that, I mean that the neck has extra harmonic resonance at certain frequencies, like everything of any mass does)at that pitch. The way to check that is to detune 5 strings and leave the E alone, and try it. I'll try that next...

Any updates on your problem?

bishopdm:
actually I changed strings Sunday and was going to check and forgot. The reason I suspect that it has to do with the neck is that I can actually feel the neck vibrating more on that note. I even wondered if it had to do with the truss rod when I first found it, but I don't think so, since it has been on two different guitars (although both Collings') I'll give it a try and let you know, but I won't be back on the site until next Tuesday, due to a short vacation. BTW, have you found any similar effect on your guitar?

No, I haven't noticed this on my guitar (OM2A). I'm guessing that if it were present, I would have heard it by now. Interesting problem, though.

bishopdm:
I've never heard it on anythng other than a dread, so you probably won't. Even though the scale length is the same. Go figure. But its not my imagination (even though my wife thinks so, but she can't even hear the difference when the humidity changes or when I change string brands, so her opinion doesn't count). Speaking of strings, its also worse with mediums than lights.
Anybody else?