I figured out how to do this while doing my Gimp adaptation of Butch Curry's "Fantasy Cartography with Adobe Photoshop" (for the written tutorial adaptation, see the corresponding thread in the Tutorial/How-To forum). The method Curry uses is fine, but I found that I didn't really like the way my forest pattern looked when I drew it by hand (I'm just using a mouse, as I don't have a Wacom tablet and stylus). The method I came up with might be a little more painstaking, but it goes fairly quickly once you learn the steps, and I think it creates a much nicer looking pattern.

1. Open a new image in Gimp. Set the Width/Height to 1 in. x 1 in. Click to expand Advanced Options. Set the resolution to 600x600 pixels. Set the colorspace to Grayscale. Click OK.
2. In the Toolbox, click the oblong select tool, and draw a circle or oval with it in the middle of your image.
3. Click Select > Distort. In the dialog that opens, keep the default Threshold. Set the Spread to 8, the Granularity to 4, and the Smooth to 2. You can experiment with different settings, but I got a good result with these. Click OK.
3. Now click Edit > Stroke Selection. I used Stroke Line, with a setting of 6 pixels. Your image will look something like this:Making a Forest pattern in Gimp and using it to paint in forested areas-alternate_pattern_method.jpg
4. Make a few more of these, following steps 2-3. Sometimes, the distortion will create little twists in your selection line, so that when you stroke the selection you get little blemishes inside or outside the shape. You can clean these up by turning off the selection (Select > None) and using the Eraser tool. To make the pattern look good, it's best to put the shapes close together, or even overlapping. Again, you can clean up the overlapping areas with the eraser.
5. Click Layer > Transform > Offset (or hit Shift + Cntrl + O). Set the X and Y values to 150 pixels, make sure it is set to Wrap Around, and then click Offset.
6. Continue making shapes in the middle of your image, and offsetting each time you've added 3-5 shapes. It should start looking like this: Making a Forest pattern in Gimp and using it to paint in forested areas-select-distort-method_treesoffset.jpg
7. As your pattern fills in, your shapes will overlap more often. Use the tips in step 4 to clean these up with the Eraser tool. You can layer them any way you like just by deciding which lines to erase. Once your pattern is filled in, click File > Save As. In the Save Image dialog, at the bottom, choose the File Type of GIMP Pattern (with a ".pat" extension). Name the pattern "Forest" or something similar. Close the image.
8. Click File > Open, and browse to <user profile>\.gimp 2.6\patterns and find the pattern file you just created.
9. As it is, the pattern is too big, so we have to scale it. Click Image > Scale Image to open the Scale Image dialog. You will see that the Width and Height are 600x600 pixels, but the resolution was reduced to 72 pixels per inch. To make this the same size as standard Gimp patterns, change the W/H size to 128x128 pixels. Then click Scale. Your end result should look something like this:Name:  Select-Distort-Pattern-Scaled.jpg
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10. You can use this pattern to easily paint in forested areas on your map. If you have a layer mask that matches the outline of your map background (such as if you created an "aged parchment" background using RobA's tutorial), you can duplicate the mask layer to do this technique.
11. Move the duplicate layer below the mask layer, and rename it "Forest layer" or something similar. Click to select the background for this layer.
12. In the Toolbox, click the Bucket Fill tool. Select Pattern Fill, and choose the Forest pattern we just created.
13. Fill the background of your forest layer with the pattern.
14. If the interior of your mask is black, you will not see the pattern at first. To see it more clearly as we do the next step, invert your mask by clicking Colors > Invert. We'll change it back in a later step. Once you can see the pattern, change the layer mode to Multiply (the layer mode setting is at the top of the Layers box). This takes out the white in the background and leaves just the pattern outline.
15. Now invert your mask again, which will obscure your pattern. Click the Paintbrush tool, and click the arrows to switch the foreground color from black to white. Now you can paint in areas of forest very easily.
16. If you zoom in, you'll see that there are some partial trees and broken lines at the edges of your forested areas. You can easily fix this by switching the foreground color back to black, and painting out those broken lines until you have a more natural tree line.

I hope you find this tutorial useful! I'll post a final map image using this technique a little later, when I've finished one.