When changing ppi, your resolution shouldn't have changed.
The ppi is just a reference number telling your computer to shove 200 pixels into an inch instead of the 72 pixels from before. The resolution of 2848x2136 should have stayed exactly the same no matter if you tell it to have only 1 ppi or 10000 ppi.
If you ended up with a different resolution, you resampled the image.
I haven't worked with CS3 yet, but if the Resize Image window is anything like CS2, then make sure you have the bottom box unchecked where it reads "Resample Image". Once you uncheck this box, no matter what you change the ppi to, the resolution (at the top of that box) will never change. Only the resulting image size in inches will change.
By resampling an image, Photoshop is CREATING pixels (or combining them) that didn't come from the camera. It might look just fine, but just realize that these pixels aren't the same as when they came from the camera.