Pre-nups aren't ironclad at all. If a judge rules that there is a significant difference between what the spouse would receive under the agreement, and what she would have received without one, he can rule the prenuptial agreement unfair and have it set aside. Since the main point of a prenuptial agreement is to ensure a spouse doesn't receive what they would otherwise have, that makes prenuptial agreements somewhat useless.
But if that's not enough for you, there are several other problems with pre-nups. For one thing, they don't protect future earnings. For another, they can get thrown out if they don't have a solid record of what they're protecting, or if a judge rules the spouse was under duress (ie, pre-nup was signed the day before the wedding rather than far in advance).
It's ridiculous that a pre-nup is even required. But even more ridiculous that it's so ineffectual.