I think the big factor in this game will be the 12th man and the pass rush. Kaepernick is a very good QB when he's left alone but disrupt him and he turns into a pile of crap- not that you'd expect much else from a QB with so little experience.
Kaepernick will need to score often to keep up with Seattle's erupting offense. I have a lot of respect for the 49ers defense, but the Cardinals have a very good defense too and two weeks ago the Seahawks massacred them. Over the last several weeks, the Seahawks have been about as deadly on offense as the New England Patriots. SF needed most of their 41 points to beat New England. It will be a similar story in this game. Unlike last time, Pete Carroll won't be content to just hand off to Lynch a lot. The Seahawks offense from week 7 looks like a single-celled paramecia in comparison to what it's evolved into in recent weeks. Kaepernick will need to score.
Seattle's secondary could potentially be down it's top 4 players at CB in a worst case scenario. That will allow Kaepernick to get away with those 1 read throws he loves to make with his very strong right arm. If Seattle can't pressure Kaepernick and the 12thman can't phase him, I could definitely see SF putting up a big crooked number on Seattle and make this game a shootout. We already know that SF can run all over Seattle's defense, although recent injuries to SF's offensive line might affect that.
My hope is that Sherman plays, our secondary steps up, and Irvin and Clemons have a field day with a QB that so far has struggled under pressure. And it doesn't get much more pressure packed than playing in Seattle as a road team. I feel very confident that Seattle will move the ball and get points. SF will probably need at least 30 points to pull this out.