You can still get in some trouble because it's federally illegal, but you don't really have to worry too much about the state prosecuting you as long as you are legal within your state. Hence, if the seeds get to you, you'll be okay. Just be smart about the whole thing. Don't go around blabbing that you got your card like some of these cats do, and then they advertise their grows and get busted and wonder why. Medical marijuana isn't a great defense in a federal court, unfortunately, even with the memos that have been released under the Obama administration.
Usually the worst thing that happens when you order seeds is they will be confiscated and you won't get 'em in the mail. Most places, like attitude, state that if you don't receive your order then they'll send you another no charge...