I'll try and find it, but the gist was that holding a planet and holding space sov are completely independent of each other. Howerver, if a Dust corp holds the Surface Command Center (bottom of the orbital elevator) of a planet, and an allied Capsuleer Corp or Alliance holds the OCC (top of it), they both receive certain bonuses.

Basically, there's benefits to cooperation, but you can't take planets to take sov away from EVE players, and they can't take your planet away from you from space.

EDIT: Keep in mind that by being able to hold planets, Dust players have the ability to put a rather uncomfortable choke hold on EVE players already. In the Tyrannis expansion, they introduced that planetary colonies would now be used to generate Starbase fuel components which were previously seeded under NPC contracts. Starbases are used in the construction of ships, ranging from T1 frigates all the way up to Super-Captials, though everything but the latter can also be made in an Outpost (null-sec station).

The big thing is that Starbases are the only means of moon mining. CCP has plans to change the places those resources are mined, but those changes won't be in for this Winter when the Open Beta launches. To better put things in perspective, the Clusterfuck Coalition, which is headed by Goonswarm, owes nearly all it's monetary power to Technetium moon mining, because that resource is essential for Tech-II ship production. A dedicated group of Dust mercs could band together to control enough of the market to be able to lock our entire coalition in a strange hold over Starbase fuel costs.

You have to understand how threatening that feels to EVE players.