What kind of mtn. bike are you getting?
Yes, I've put a mtn bike in the back seat of the STi before. But you have to take both wheels off and put them in the trunk. You also have to cover the chain with a shop rag so you don't get grease on your seats.
And even after you remove both tires, you still have to phenagle the frame into the car. Be careful, your handlebar will want to tap against the window or rear windshield and the axle mounts on the front fork will want to poke the back of your front seats. It can be done, but you have to take special precautions to avoid ripped or stained seats.
Now, if you want to go one step further, you can loosen your headset and remove the handlebars from it and be able to put the frame and both wheels (which still have to be removed) into your trunk.