In the following code, only the second method works for me (.NET 4.0). FormStartPosition.CenterParent does not center the child form over its parent.
Why?
JYelton's answer worked for me, but the form is only centered the first time Show() is called. If you want to Hide() the form, and then have it re-centered on the parent every time Show() is called you need use the following in your form:
public new void Show(IWin32Window owner)
{
base.Show(owner);
if (Owner != null)
Location = new Point(Owner.Location.X + Owner.Width / 2 - Width / 2,
Owner.Location.Y + Owner.Height / 2 - Height / 2);
}