Personally, I'd go with Anna's frames. Quick & easy
If you want to learn how to do it yourself and have some flexibility, you might want to check out this tutorial. By adjusting some of the values I give, you can get that effect.
You would use the vignetting section of this tutorial:
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Cassie
My Gear: Nikon D300s w/18-200mm VR & 50mm 1.4
Software: Adobe Photoshop CS4, PSE 10, Lightroom 2
