Hmmmm... everyone on here seems to think the secrecy must be because the government is worried about "the public" finding out about horrific terms. That seems unlikely--remember, IP law doesn't even make the top ten of most US voters' important issues. War, health care, income taxes, education, research, crime, terrorism, etc... all trump IP law. So a politician's concern over public negotiations isn't likely to be that it may trigger some vague public discontent. The politician's main concern is that a corporation that cares *immensely* about copyright law will find out that something proposed in the treaty isn't to their liking, and then spend a ton of money to remove that politician from office before the treaty is finalized. Different wealthy corporations have different goals for copyright law (think Google vs. Publishers) and balancing them is probably impossible without making many very mad.
The treaty might be good, might be bad, and there are lots of reasons to be against secret negotiations (remember, the final treaty has to be presented and voted in public). But assuming that secrecy means the end product MUST be bad seems unfounded. Think of it this way: if you were in charge of the negotiations, and wanted to write the most Slashdot-friendly IP treaty possible, you would HAVE to keep negotiations secret. Otherwise the RIAA et al. would spearhead a $10B campaign calling you soft on crime, mean to elderly people, etc, etc..., removing you from office before the treaty could ever be passed.