> This performance optimization doesn't really work for variable-width blurs...
An alternative would be to use Summed-area tables [1] where each texel contains the sum of all texels that are above and to the left of the current texel. This allows variable-width blurs to be computed in constant time, using only four texture samples (!).
And the concept of a variable-width blur is an approximation to a related concept in photography/optics called the "circle of confusion." [2] The circle of confusion, combined with bokeh sprites, are usually what AAA games are doing for depth of field [3].
See [4] for more details.
[1] http://http.developer.nvidia.com/GPUGems3/gpugems3_ch08.html
[2] http://en.wikipedia.org/wiki/Circle_of_confusion