Hmm, am I missing the description of the Tome Dragon's precognition ability somewhere? I've been flipping back and forth and can't seem to determine what this ability does...
I haven't received my copy of the issue yet, but as the author of the arcane dragon article... huh... looks like I left that out of my original manuscript too! Guess that slipped past the proof readers. Anyway, it's just a justification for the tome dragon having an insight bonus to AC (a 'spidey sense' kind of deal). This isn't official, but I'd say the precognition ability should read something like:
Precognition (Su): The tome dragon's mastery of divination magic allows it to peer a moment into the future. Knowing in advance where a blow will land, the tome dragon can easily evade the attack. A tome dragon receives an insight bonus to Armor Class that improves as the dragon ages.
'Arcane Dragon' is indeed a class of creatures like chromatic or gem dragons. Or, for that matter, like baatezu or eladrin, with certain traits common to all monsters of that (sub)type.