Hmm, I don't think it's necessary to have Blomkvist to relate to, as such- I think cinema puts too high a premium on having a "relatable" (read: "boring") hero the audience can identify with; this film actually would work rather well as an interesting character portrait of Salander, who was magnificently cast, I have to say. (They actually went butcher than the book! I love the swedes.)

However, as it is a murder mystery I think Blomkvist is actually essential; much more so in the film than the book for the simple reason that Salander is so undemonstrative and uncommunicative, the audience wouldn't understand her thought processes in the slightest if she didn't have to explain them to a straigt man. So Blomkvist is the Watson, the House-ettes, the layperson the genius can explain their insights to.

Besides, I don't see him as the hero of this film at all, really. Lisbeth steals the show, though she really only steps into play later on. And I quite like how girly Blomkvist gets about her, wanting to share a bed and play with her hair and talk about the future. They're an interesting duo, in which Salander is decidedly the Batman.