Greg Oden is the only player I'd deem untouchable.
Brandon Roy and LaMarcus Aldridge I would be willing to trade, but it would take pretty huge returns. They are almost certainly untradeable, because the players I'd want back are ones who are likely untouchable to their teams. I value Brandon Roy higher of the two.
Jerryd Bayless and Rudy Fernandez are both in the "blue chip prospect" category. They are tradeable, but would require other excellent talents in return, like a Danny Granger or Josh Smith type of player. I am not right now evaluating whether they are identical in value to those players (I could imagine Portland adding more to their side of the trade) but the point is they are definitely tradeable, however I'd want a lot back.
Travis Outlaw is very tradeable to me. It's hard to describe what I'd want back for him. I think he has a fair amount of value, but Portland probably can't leverage his full value themselves. So I'd want a decent return, but not a star. A solid, young starter-quality player or prospect with starter upside would be sufficient. I consider Outlaw the primary trading chip on the team and would be willing to package him with a Bayless or Fernandez for a young, star-level talent (Granger, say...).
Everyone else falls into the "mix-and-match" category. If other teams are interested and there's a trade that makes sense, go for it. They're of varying value, but none are likely core players or key championship-level support players.