Er...so, increased sales how? From people who didn't play (thus likely don't have) Oblivion but plan to get Skyrim downloading a DLC for Oblivion? There's a less-than-minor flaw with that plan...
I would think there will be people new to the series who might decide to pickup and play Oblivion while waiting for Skyrims release. Im no marketer but Id also imagine anyone whose hyped up for Skyrim who does not already own Oblivion would probably be encouraged to get it to play any kind of Skyrim related stuff(like the books).