I recently had a need to use fill pattern in OmniGraffle 3.1.2. The problem is that OminGraffle 3.1.2 doesn't come with fill pattern support. However, I figured out how to import fill patterns from Microsoft Powerpoint to OmniGraffle 3.1.2.
Go to PowerPoint, create a simple shape such as circle, rectangle, square -- really, any shape will do. Control-click on the object and click "Format auto shape." Click on the "Colors and lines" tab. Click on "Fill effects...," and then on the "Pattern" tab. Click on a pattern, and select your Foreground and background. Click "OK" when you are done.
Now just copy the shape with the filled pattern from PowerPoint over to OmniGraffle. Once you do this, you have an array of options in OmniGraffle, since it is treated as an image. For example, you can change the shape to anything in the shape pallete. Since it is an image, you can also adjust the opacity in the image pallete, etc. You can shadow it with the shadow pallete, etc.
[robg adds: Among other cool features, the Pro version of OmniGraffle has fill pattern support. However, it's also $50 more than the standard version, and if all you need is a fill pattern, this is a good workaround.]
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http://hints.macworld.com/article.php?story=20041130115202479