DIY OnlineMag.com: They have several upholstery tutorials and handy guides including upholstery terminology, types of tufting, etc.
DIY Network: Has several upholstery repair and re-upholstery tutorials.
As you know, the fabric is a hugely important part of your re-upholstery project.
Do it Yourself has great guide to understand the difference between the different types of fabrics available. And if you are looking for some great fabric sources check out Upholstery Obsession: Online Sources for Home Décor Fabric
If you want to take a look at the amazing things you can achieve with new fabric, take a look at some of the Best Before and After Upholstery Projects of 2011 Rescued & Revived: Upholstered Furniture Before & Afters, or click over to The Divine Chair, to see some amazing pieces.
(Image: Little Miss Penny Wenny)