Just want to add a couple of things that I think have a place here.
Firstly, at the White Dwarf/Design Studio seminar at Games Day UK, Wade Pryce said that the same thing that triggered full-colour, hardback books for Warhammer Fantasy will trigger full-colour, hardback books for Warhammer 40,000. To me, this quite clearly means, come 6th ed, 40k will be treated to the same, lovely, style of books as Fantasy has been receiving since 8th ed.
Secondly, Imperial Armour Apocalypse Second Edition features quite a few units that used to be Apocalypse only and are now legal in games of 40k. This backs up the rumour of 'a book to rule all books' and the hints that come 6th ed, regular games of 40k will look a little more like Apocalypse games.