Umm, so I was just looking at these and I had a few thoughts I wanted to share.
PHB- Has anyone officially stated anywhere that the wizard/sorcerer looking horned character on the PHB cover is in fact a Tiefling? Because he doesn't look like one at all. I know that its said tieflings will be core in 4E, but that doesn't make that particular character a tiefling. You know what he looks like?
A half red dragon.
Tinged red skin, red dragon horns, a dragon tail with a frill, it looks like he has talons on his casting hand, he has ridges on his cheek bone like a red dragon does, and even has a point on his chin just like a red dragon does.
DMG- A red dragon is on the MM cover, looking in his/her crystal ball on the activities of the two characters from the PHB cover. Is that horned mage in fact this dragon's son/descendant? I'd be willing to put money on it.
Its also worth noting that the dragon is looking in on PCs, plotting and probably manipulating their lives. Which when you think about it is exactly what a DM does, and thus makes the cover excellent in terms of reflecting the books content.
MM- The MM cover looks out of place with the other two, which is why I imagine that isn't going to be the final cover. This is all conjecture, but I imagine the final cover will depict whatever creatures the characters on the cover of the PHB are reacting to, with a lightening strike arcing into the frame.
I think that Orcus picture is great, but may be used somewhere else.
So I think that all three covers will in fact have an interrelation and do in fact reflect the contents of the books themselves.
I apologize if someone else has already said this.