Figured that even though they won't pre-install an OS on it they'd atleast support the kernel on it. Like provide assistance on which modules to load and support on the hardware they ship. Like "This hardware needs this application" or "This hardware doesn't do this yet" and things like that.
As long as I know Dell computers don't have any Dell proprietary hardware so I don't think they'll develop drivers for the hardware but I think that they will instead avoid making available OS-less versions of laptops that lacks the needed linux drivers to get linux distros to work on them.
The certification probably will mean that the distro will be certified to be compatible with those laptops (and nobody knows if dell will guarantee that the "extra" laptop features will work on linux).
Edit: So Dell is esentially doing what they've been doing for years now, sending out clean boxes, but they call it "Linux certified"? Amazing spin-doctoring.
From their announcement it looks that they're going to do exactly that.
The reason is that it's probably not convenient for any big hardware manifacturer to sell desktops computers running linux: linux can run with lower hardware requirements (there are distros that scale very well on old hardware (no, ubuntu is not one of those)) and this means fewer new hardware sales for them (people would upgrade less often).
The dream of the hardware producers is to (like Apple does) force you to regularly buy a new computer: windows lets them do that (because windows usually has always higher requirements than the previous versions), with linux they can't.