I don't know that I'd call it a rivalry. For it to be a rivalry, both teams have to meet up fairly regularly when they're at the top of their games and contending for a chance to win a title. That happened in the early 90's and in 2000, but since then the Blazers have been in rebuilding mode. I think it has the chance to become a legitimate rivalry in the next few seasons as the Blazers are set to really start to make an impact at the same time as the Lakers have regrouped with a solid core of players around Kobe.
As a fan, I know that there's no team in the league I enjoy seeing the Blazers beat than the Lakers. It's a combination of factors: Kobe's general arrogance, the glitz factor, the undeniable fact of the Lakers' longterm success, and maybe a bit of a sense of entitlement in Laker fans in general (present company included.)
It's going to be fun kicking the Lakers off of their perch in a few years.