I booked my passholder discount online. Go to start your purchase, then there is a field on the top of the screen for a discount code. Enter passmember2011, and the available dates for the passholders will show then, once you enter your # of guests and date range.
When you check in, you'll have to show your pass. I got to Orlando last Friday (the 18th) and checked the following day. Glad I did - the only days left were Wednesday and Thursday. We chose Wednesday, which was perfect - 88 degrees and sunny! Thursday was warm too, but very windy.
You do not get admission to any other park - with our Platinum pass that would be redundant, but with any pass you don't.
I don't know about the "buy a day, get a year" pass for discounts.
The platinum pass (super grover pass) saved us a lot - 10% off everything, even beer at Seaworld, and merchandise at DC (and the photo CD), plus free preferred parking at Seaworld.