Totally agree with Nick – I really like Illustrator for diagramming because of the ease of drawing and manipulating lines. It can also be a decent program for doing plan graphics, although the files tend to get really large. The added benefit of using Illustrator is that that you can open .pdfs from AutoCAD in Illustrator and work directly with the lines from AutoCAD (vs. Photoshop which rasterizes the image first). You can also export to other programs (as mentioned in the post below), including to a DWG. The conversion is not perfect, but gives you a good start.