You can use the rounded rectangle tool or the ellipse tool in PS to create round-ness, and just crop the areas you need; or you can do what i do, and just make the rounded corner yourself by using pixels (I do this especially if I need the outter area of the corner to be transparent). I just use the pencil tool on 1px and draw and mess with the circle shape until it looks good. (Then, just duplicate it and rotate it to suit all four corners). Here's an example I used from a website I created quite a while ago... the border of the site was 5px so i needed to create a corner that would fit:

...It actually looks a little jaggidy here
But it looked good when put together with the rest of the site.
HmMmMm... maybe I should have used a better example...
Any way that's how I do it.

