After doing a bit of research, I'm happy to say I'm not the only one who has been frustrated by Photoshop's strange lack of options when it comes to creating a simple white border on your elements. I've tried stroke, expand, and even "border", but all of them are either rounded or feathered, and it makes me crazy! Finally, I found something that works (other than layering a disproportionate white rectangle underneathe).
Here's the step by step on how to get a simple square border (at least in PS3).
Activate the layer. (Click the thumbnail in the layers palette while holding the Ctrl key)
Hold down the Shift key and the letter "Q"
From the top menu, select Filter> Other> Maximum
Choose the width of your border
Shift-Q again
Now you'll see an empty marquee around your picture, fill that marquee with whatever color you want for your border.
Now hit Ctrl-D on your keyboard to get rid of the marching ants and there ya go!
It may seem like a lot of steps, but after you've done it a couple of times, it's really quick and easy.