I'm not sure how to describe this problem, but here goes--my model looks all nice and shaded when it has a regular lambert shader on it, but as soon as I try to load a file (a psd file that I painted to line up with my UV maps), the shading on the model goes away & it just looks like a 2D-flat-solid color; no matter which way you tumble it, no shading. Is there some switch or slider I need to have turned on/off in order to show shading on my model in the viewport? Why is this happening?
Nevermind, I figured it out. In the viewport, under Shading --> Hardware Texturing --> options box, I had it set to 'over' instead of 'multiply'. Maybe I just hit a key that happened to be a hotkey for this? Weird.