What did you mean by I would lose my shirt?) I know im probably down 3-4 g's now but I would imagine you could get a used sti ( if I could find it) for about 14/15?
Unless you bought the 09 outright, which most people can't afford. You got a loan for 25K or whatever the car costs. The second you drive it off the lot, it probably dropped in value to 21-22K. You still own 25K on it though. You trade it in, you're going to have to come up with 2-4K out of pocket to buy yourself out of your loan. That's "being underwater". It's going to take 3 year normally for the value of the car to match your loan amount so, you can trade it in or sell it for what you owe.
Lets say tomorrow the dealer got a 04 STI on trade. He's paying 12-15K (I didn't look it up) for it and is going to turn it around for 21-22K. Lets say you want to buy it. You can trade in your car and pay the dealer another 4K to cover the loan (Loan amount - what the car is worth = cash you have to pay out), then you have to get a new loan for 21-22K to buy the STI.
You might be able to roll the negative equity into the new loan, but, then you're paying 25K for an STI that's worth 21 and you're in the hole again.
If you bought the car with cash, this is a non-issue. You trade it in, eat the 4K loss in value and buy the STI.