Actually it does have Portland, assuming you are in Oregon. (There are other Portlands). I would recommend hitting www.zagat.com
and using the 'change location' near the top and then 'see all locations' to see what is potentially covered. I say potentially because the web says there are 218 entries in Portland,173 are resturants, and the iPhone app indicates 90 resturants. So it doesn't appear that the app is as complete as the web site. Consider that the web site is a subscription costing $2 a month, it figures.
The big catch is that the iPhone app only appears to support the 'Zagat' reviewed establishments, while the web also has member entries. In my town that changes the resturant entries from 59 on the web, to 9 on the iPhone. Only 2 of the 9 are not chains. Nothing wrong with chains, but if you've been to one, you've been to them all. Don't really need a review, and if I want to go to one I can find them in Maps, or in the yellow pages, on my iPhone for free.
So, while the app is a nicely done application, the data appears to be highly dependent on whether you live, or travel to, where a lot of people live.