The crop tool does not resample unless you have a number entered in the resolution box. If you are getting a different result, it isn't a Photoshop problem.
This is not quite true. It's a bit more complicated. In fact, in Photoshop CS4 and CS5 (at least), the crop tool does
resample your image if you enter a width and height in pixels
, whether or not you have anything in the resolution box.
Furthermore, if you use a unit of measurement that would result in an image with a resolution higher than 9999, then your image will also be resampled to the targeted aspect ratio at 9999 PPI. Why this particular number, I do not know, but it is consistently the case. You can test this yourself by specifying width and height in millimeters or points, and then cropping a large portion of an image.
The solution is fairly simple: If you have your ruler set to pixels, always specify a unit of measurement when entering a ratio for the crop tool, in inches ("in"). For example: "3 in" x "2 in". No resampling will occur because the PPI will not climb over 9999 unless your image is positively massive. The width and height will be set at the size you specify, and resolution will be adjusted to fit that width and height. You can then reset the resolution using "Image Size..." and let width and height fall where they will.