For Terragen 3, we have developed a proprietary new method for fast, noise-free Depth of Field. Unbiased DoF is always available, but in most cases you can achieve realistic results much faster using our new approach. Below is a sample of the results, rendered in 37 minutes on a standard Core i7 3770k at 3.5Ghz. Without DoF, the same scene rendered in 32 minutes.
Update: Some more Terragen 3 Depth of Field images to enjoy. Thanks to Jeff Boser, Martin Huisman, and Ulco Glimmerveen for these scenes.