The long-running Bwin-Harvard study covers more than just poker.
This was the only thing I found other than poker or sports betting:
A Harvard Medical School study into the habits of thousands of Bwin customers found that 5 percent of online casino players demonstrated signs of problematic gambling behaviour, compared with just 1 percent of sports bettors in a previous study. Bwin says the findings will assist in the launching of an automated warning system over the coming months, but they may also face political repercussions in a pan-European context.
And you cant call it totally impartial. And it has only focused on one site and one software.
So I wouldnt say that online gambling isnt more addictive than land based gambling based on a study that has mainly focused on poker players.