Be careful that we don't let the secret out. No everyone, Portland really sucks, you wouldn't want to move here. Try Boulder maybe.
There is one really noteworthy, non-race team oriented riding club in Portland, its PUMP (Portland United Mountain Peddlers) I've known lots of great folks that ride together under that banner. You can find them on Yahoo or at the club site.
I just got home from a brief afternoon ride up on Powell Butte. Great local singletrack for the close in SE rider.
Tuesday and/or Wednesday afternoon or evening I try to get up there. It's close in and if I get the rare chance to ride, that's where I head.
Sometimes I ride Forest Park. That's a great place to do longer distance mtb rides. It sometimes feels more like a road ride.
Enjoy the fall, the rains are a brewing!