Irregular shaped Windows Form (C#)

@Geoff shows the right way in winforms.

But If you are planning to use WPF instead of Winforms then WPF(.NET3.0+) gives very flexible ways to create anyshape custom windows. Check out this article also http://www.codeproject.com/KB/WPF/wpfpopup.aspx

this.BackgroundImage = //Image
this.FormBorderStyle = FormBorderStyle.None;
this.Width = this.BackgroundImage.Width;
this.Height = this.BackgroundImage.Height;
this.TransparencyKey = Color.FromArgb(0, 255, 0); //Contrast Color

This allows you to create a form based on an image, and use transparency index to make it seem as though the form is not rectangular.