Now that Velonews.com has posted the complete data for the wind tunnel test of disc brakes versus rim brakes, it's time to take a look at the time gaps you could expect to see across the mythical 40k TT.
As you see in the Velonews graph, the speed difference between the disc bike and the rim bike is greatest when the wind is blowing from the drive side of the bike. The drive side of the bike is normally the faster side of a bike with rim brakes. The dynamic that causes this is also nearly certainly responsible for this difference - to put it in the simplest terms we can, you're adding sail area to the disc side. This helps to overcome some of the drag added by the disc setup, and makes the disc side of the disc bike relatively faster than the drive side of the disc bike.
However, this difference is most profound at greater wind angles - wind angles that don't commonly come into play. There have been a bunch of studies that show that even in fairly windy locations, at the speeds we typically ride, we are most often riding with an apparent wind angle of 10* or less. This study that Trek did while developing their SpeedConcept bike is a great read on this (section 3.2 has what you're looking for). Our convention has been to use the weighting that Tour Magazine uses for their tests, which places a similar emphasis on wind angles of 10* or less, but logically brings wider angles into play at slower bike speeds. The slower you go, the more likely you are to experience wider wind angles, although wind angles of greater than 12* or so are still quite rare.
Between -10 degrees (the drive side is the negative side) and +10 degrees, the extra watts of drag that the disc bike incurs are fairly symmetrical. As you get out to the wider angles, the drive side becomes relatively slower, while the non-drive side becomes very equal to the rim-braked counterpart.
Thanks to this distribution, the delta between the disc and rim bikes in the mythical 40k TT are similar no matter which side the wind is from. Imagining a flat, out-and-back 40k TT course where the wind is blowing from the left on the way out and the right on the way in, the time costs for the disc brake bike are:
@30mph - 3.6 seconds out, 4.7 seconds in - 8.3 seconds total
@25mph - 4.5 seconds out, 4.5 seconds in - 9 seconds total
@20mph - 3.5 seconds out, 4.5 seconds in - 8 seconds total
My New Years resolution (I haven't made one in about a dozen years) is to use subjective language as infrequently as possible, so we will simply let those numbers speak to themselves. We're sure that the forums will host fierce battles for all perspectives. Our objective (see what I did there?) in doing this exercise was merely to quantify the difference between the two setups in order to give people the information that allows them to make the decision that best suits their purpose.