Yes, when demand drops (or supply increases), the market responds accordingly. The good news for a city like Portland is that the sharp spike in apartment construction will eventually mean smaller rent increases, or even rent drops if inventory significantly exceeds demand. The interesting discussion is why Portland's not-quite-stellar economy is generating high population inflows. The New York Times had a piece about this phenomenon a couple of weeks ago:
http://www.nytimes.com/2014/09/21/ma...lbias%3Ar&_r=0
Great cities cost a great deal of money to live in. Portland is still known as the cheapest city on the west coast but it is being discovered, particularly by people who are giving up on costlier cities like Seattle or San Francisco. If Portland didn't have the cachet it does, then it would be less attractive not only to newcomers but residents as well. Call it the dark side of success but I think most people understand the trade-off involved. If Portland's economy were as hot as Seattle's or San Francisco's and the wage scale was correspondingly higher, then rents would be even more exorbitant. The good news is that many of the newbies are not suburbanites but urban warriors who want to live in the city. This means greater density and all the good things that derive from it - better mass transit, better restaurants, more art and music venues, etc. The down side, possibly, is a kind of homogenization of the city over time as yuppies replace hippies. It's my one real concern about Portland's success. And true enough, Seattle and San Francisco led the way on that front, too.