Update! Simple tutorial here: http://elementalray.wordpress.com/2012/05/07/the-user_ibl-shaders-pt1/
Quick n’ dirty usage:
connect the user_ibl_env to the custom shader slot of an area light
-turn on visible
-set samples (this is a literal value) I can get away with 4-8 using Unified Sampling and a basic daylight HDR, you may need more for a complex HDR
Now connect the user_ibl_env to the Environment slot of the camera. (you can still use the rayswitches and envblur with this shader.)
The samples (high AND low) of the area light must match the samples of the user_ibl_env shader or you may truncate the importance sampling. It’s easiest to just connect the high and low samples to the user_ibl_env samples attribute so they stay the same.
You must use the “light relative scale” string option with a value of 0.318 (1/pi) to correctly light materials, this shader is designed for BSDF so old materials will blow out some if this isn’t set.
Blog post coming at some point.
Additionally: When lighting with this, use a filter on your Final Gathering and greatly reduce the accuracy. Only secondary bounces are calculated by FG when using the user_ibl_env. You can get away with values that are low, in my case 32 rays provides plenty.