Recently, I struggled with Adobe Photoshop CS2's Shape Tool. I was disappointed to discover that my shape tool (rectangle in this case) when in the fill layer setting, was automatically anti-aliasing all my edges (not just the rounded corners as expected). Two other web developers have confirmed the same results in CS3 beta as well. Crisp in Fill Pixels (with option to antialias), but fill layers refused to draw crisply.
Solution: It appears that CS2 has the unexpected behavior that drawing with shape tools on fill layers when zoomed other than 100% causes the fill layer mask to automatically anti-alias! Zoom back out to 100%, and the shape draws "crisp" as expected!
I wrote about it in more depth on my website along with a sample image, and I have reported this bug to Adobe but have heard nothing back since. If anyone else can reproduce the same result or requires more info for the same, please post here (and on Adobe's bug report tool).