This seems like a pretty basic function, but my copy of Photoshop can't do it right. Specifically, I'm using CS4, but I have a feeling that doesn't matter. I'd appreciate it if someone else would check to see if their copy of Photoshop works correctly either. Here's some quick step-by-step instructions to try it out:
Step 1 - Create a new file that is 4 pixels wide by 4 pixels tall. (Use basic options: 72 pixels/inch, RGB Color, Square Pixels, etc.)
Step 2 - Using the pencil tool, draw a red dot (1 pixel wide/tall) in each corner. So that the image looks like this: 
Step 3 - In the menu, go to "Image" > "Image Size..." and increase the width and height of the image to 400 pixels using "Nearest Neighbor".
Now here's the image I end up with:
So we started with a 4x4 image, and now we have a 400x400 image. The math here isn't too difficult: 4 x 100 = 400. So each of our red dots in the corners should now be 100x100 pixels each. Simple enough, right?
Unfortunately, this was not the case for me. The red dot in the upper-left corner is now 97x97 pixels. The red dot in the upper-right corner is now 103x97 pixels. The red dot in the lower-left corner is now 97x103 pixels. And the red dot in the lower-right corner is now 103x103 pixels.
I guess Photoshop seems to think that both of the following statements are simultaneously true:
1 x 100 = 97
1 x 100 = 103
Am I alone in experiencing this buggy behavior? Or does everyone else get the same result? Is there some way to fix it?
Thanks for giving it a try!