Is shape change automatic? For instance, if an attack is declared and the opponent with shape change stares blankly into the abyss for some time, is it then legal to continue with your roll, believing that because he has forgotten his own figure's ability that in essence he is choosing not to use it?

Technically you can not choose to not use Shape Change. Legally speaking, you should ask him to roll his Shape Change before making the attack. Because if you attack before he makes the roll, you are cheating just as much as he is.

So, in order to not use shape change, he'd either have to be negated the privelege or be committing an illegal act.

Actually, I was wrong.
It says that your opponent may make the dice roll. But it has always been explained to me that you should remind your opponent to roll for it. It's just good form.

Quote the power, please

SHAPE CHANGE When this character is chosen as the target of an attack, you may roll a d6. On a result of 5 or 6, the attacker can’t target this character with an attack this turn and the attacker may choose another target character instead. The attacker then makes the attack unless there are no targets, in which case the action resolves without an attack.

Shape Change includes the word may now. It is optional in its use

So if your opponent is aware that his fig is being attacked and you have declared it and he just stares blankly, you may, after a reasonable amount of time, continue

Allowed, but not honorable got it. I think that it's cheap but at the same time fair. May not be quite parallel, but I would not at all want to remind my opponent to outwit my RCE or to perplex his defense up by one, though I have been courteous enough to do so before...