The latest alpha build of Paint.NET v3.5 is using DirectWrite instead of GDI for the Text tool if you are running Windows 7.  More details and a pointer to download Paint.NET v3.5 that uses DirectWrite is available here. 

Paint.NET is a graphics editing program for Windows, developed on the .NET Framework. Originally created as a Washington State University student project, Paint.NET has evolved into a powerful graphics editor with support for layers, blending, transparency, and plugins.