The info I'll put here applies generally to anyone interesting in doing street photography in the U.S., in Portland and elsewhere.
Portland residents, like those in Seattle and San Francisco, are a lot less uptight than people in larger Eastern U.S. Cities. Maybe it's the close proximity to the Pacific Ocean. <G>. So if someone spots you photographing them, you might suddenly see them posing for you and hamming it up. Still, there are the basic rules that ArtOn nicely summed up above.
As a photojournalist, I've done a fair amount of street photography for publication with and without getting releases. Two informal rules I tend to work by are: Courtesy will oftentimes get you better images than sniping. If I happen to see someone who appears photographically interesting to me, I engage them and ASK if they would permit me to photograph them even if it's not for publication. This is especially true of people who appear homeless and when you want to photograph a kid who appears younger than 18 years. I generally introduce myself, very briefly explain what I'm doing and tell them why I think recording their image is interesting or important. I may give them a card and ask them to sign a model release. Here's some excellent editorial info. on public photography from the New York Times Magazine in April of this year. The images, of course, are great too.
A lot of info on street photography law is based on anecdotes or wishful thinking rather than the law itself. A few years ago there was a guy running all over the net publishing booklets for sale with legal info that he believed was correct when in fact a lot of it was not. So as you read about this interesting subject, pick the sources you want to rely upon wisely.
Some such reliable ones include (without limitation of course) :
One from a lawyer named Bert Krages who, last I checked, is located in Portland and he does intellectual property law that applies to photographers and copyright issues. Here's a really useful single page you should print out and carry with you:
http://www.krages.com/phoright.htm and the form located here: http://www.krages.com/phoright.htm
It generally covers the rights of photographers in public places here in the U.S. and since he's a lawyer, I'm reasonably certain it's quite up to date. His website also has a lot of useful info for street shooters. http://www.krages.com/bpkphoto.htm He's also written a book entitled "Legal Handbook for Photographers: "The rights and liabilities of making images. Published by Amherst Media.
Another solid source of info is by Leonard Duboff, another lawyer, also in Portland. His book is called:
"The Law (In Plain English) For Phographers"
Victor S Perlman is an intellectual property lawyer AND happens to be the lawyer for ASMP in NY. He wrote an article called "Legal Perils on the Street" in the April 2013 issue of Rangefinder Magazine that you can probably find online. Perlman takes a common sense approach to these issues and I agree with his philosophy. Know your basic photographic rights and it's both just and proper to "...keep them from getting trampled" but it requires a cost-benefit analysis that are sometimes outweighed in favor of the potential costs involved in litigation over use/misuse of an image.
Finally, I suspect you know that photographic rules have changed substantially here since 9/11. For awhile, state and federal cops and security people, officials and others, stirred up a real paranoia over photographers just making photographs of public buildings, structures, monuments, ad infinitim. In San Francisco for awhile, it seemed everyone with a camera, tourists and photojournalists included, were immediately suspected of plotting terrorist acts for things like taking a picture of a subway train.
It's simmered down quite a bit but be mindful (not necessarily paranoid). Cops often use photography as a pretext to stop and ask for ID. It's BS but they are entitled if they have "a reasonable suspicion" of foul play afoot.