Well, insurance in the US is a pretty tricky world to navigate, made only more tricky by being trans, but as much as insurance companies will be "the bad guy" to make money, they won't do anything illegal. Essentially, your best weapon is understanding.
First of all, your being open and honest in all medical situations, while potentially devastating in the past, is exactly the right move now! Part of the Health Care Reform bill was that, starting in 2010, insurance companies are no longer allowed to rescind coverage due to incorrect information unless the statement was fraudulent or intentionally misleading. The most misleading aspect of hiding a condition would likely be fear that you would not be eligible for insurance due to a pre-existing condition, which in turn is being abolished starting in 2014, also as part of the Health Care Reform bill. So, your providing that information will not allow your insurance company to rescind coverage, especially since the pre-existing nature of gender identity disorder is itself highly debatable. Yes, I knew from an early age that I was the wrong gender, but I didn't know it was a medical condition, there's no check-box for it in medical history forms, and like most of us, I spent years understanding and coming to grips with it.
So, to be clear, whatever you do, do not ask the clinic to intentionally provide fraudulent information to the insurance company, as it is the ONLY way they will legally be allowed to rescind coverage and if they're allowed to, they will.
Being upfront with my insurance company has been great. I called and asked about in-network therapists who specialize in GID and was given a list. Other than therapy, the only thing I know that most insurance companies cover for transition is hormones, but we need all the help we can get, right?