vapodog,
There have been several articles written on the 400 and 465 since the announcement about a year or more ago. Jim Dodd has been in contact with H&H during testing and proofing of the new cartridges. I believe Jim wrote an article for African Hunter last year giving more specifics on the concept, design, and testing of these two.

Basically what H&H wanted to do with the 400 is give it the same effect on game at 100 yards that the 375 H&H has at the muzzle. The 400 H&H has a long neck and very sloped shoulders designed so the 400 grain 400 Woodleighs would not protrude into the powder space and pressures be kept down.

Whether the cartridge does as H&H claims or if it will ever see any popularity is for more brilliant minds than mine and nimbler tongues. It would have been an interesting cartridge so many decades earlier, but does not really offer anything that isn't already covered by the 416 Remington, except to use .411" bullets which are more limited in variety and often harder to comeby.