These steps can be used for rounding shapes or anti-aliasing a jagged layer.
Create a new image, and on a new layer, fill in the shapes you want rounded, or use the Type tool to draw a letter.
Select the layer transparency of the layer you want rounded, by holding down Ctrl and clicking on the layer.
On the Keyboard, press Q to enter Quick Mask Mode.
Filter > Blur > Gaussian Blur... 6 pixels
The number of pixels you blur it by determines the radius of your rounded corners.
Image > Adjust > Brightness/Contrast
Set the Contrast to about 94%, and then change the Brightness to whatever looks good.
Raising the Brightness will make the objects thicker, and lowering will make them thinner.
Now, press Q again to exit Quick Mask Mode, and create a new layer.
Press Alt+Backspace to fill the new layer with the foreground, and then delete the original layer.