Hey guys, I thought this was going to be really easy, but now that I'm trying to do it, it seems a bit more difficult (at least to me) than first thought.  I render cars.  And all that I want to do is simply reflect the car into the ground plane.  In essence to create the effect that the car is on a mirrored or shiny surface.  I thought I would just copy the image and then flip it vertically, and then adjust the opacity, etc.  But, I'm realizing that just simply flipping it won't work, unless the car is is situated completely flat along the the horizontal.  If it's in any sort of perspective, this won't work.  Any help you could provide would be appreciated.  Thanks,

In Photoshop what you need to do is, adjust the “flipped” image perspective to match the car’s perspective.
To do that, use the “transform” tools like Skew and Perspective (if you have Photoshop CS2, the “Warp” transform tool and “Lens Correction” filter could also be very useful).
Once you get the perspective’s to look right, try using the “glass” filter at a low setting to get a reflective look.

However you said, “I render cars”…
If by “render” you mean the image of the car was rendered in a 3D app, it would be much easier to do the reflection in your 3D app, (just put a plane under the car with a reflective material applied).

 SWAMP