So, given that the rivalry's so fresh and new, I'm not expecting stories as juicy as the Raiders stories. But, I have a few;
2009 - While a bunch of us we're in SF, MysterMatt scaled a SF 49er Billboard and PLASTERED it with Seahawks.net stickers. Greatness (please note this was before Matt had long hair, so, it was better)
Umm, 2004...maybe? - Whenever the sharpie game occurred, I had my first unfortunate run-in with a 49er fan. It was in the Hawks nest, featuring two mediocre teams. This 49er fan was hollering and baiting us all game long. So, on his way back to his seat, nacho's and beer in hand, I "accidently" did a touchdown signal with him walking behind me. Sadly, two beers and nacho's soiled his clothing before eventually hitting the ground. Eventually dude's 49er hat got tossed on the field, by some other loud nitwit. Pretty great and immature.
You got any?