I maintain a thread on the Visual Arts section of a writing site where I post "mini" tutorials for backgrounds for sigs and such, but many of them turned out to be longer than expected, so I'm going to post them one by one. Please let me know if you find any errors (I tend to mess up on tutorials) or if I need to clarify something.
1.) Create a new image with a white background. I usually do mine at 550x200, 500x150, or 350x125.
2.) Select the gradient tool and set it to biliniar, difference mode, and the rest of the default settings. Drag a long line in a random direction on your image. Repeat four or five times, until you have a line in each general area of your image.
3.) Go to Layer > Colors > Levels. On Output Levels drag the arrows inward (the left one a lot more than the right one). You want it too look like a pretty smooth grey (and still be able to see the contrast okay).
4.) Go to Filers > Distorts > IWarp. Set the deform amount somewhere between 5 and 10, and make the radius at least 30. This may vary slightly, depending on the size of your image and distance between your lines. Set it to one of the Swirl modes. Move your mouse slowly around the mini-image preview it gives you. Switch to the other Swirl mode. Do the same thing, but in different spots. Don't distort it too much, or it won't look right. If you mess up, you can hit Reset to begin again.
5.) This step is optional, depending on what you want it to look like. Filters > Blur > Gaussian Blur of 5-10.