Thanks to land use planning guidelines still in effect (newly under attack by developers), rural development has been kept under check compared to most metro areas around the country.
Going west, you'd probably find something around Hillsboro or North Plains. Going east, maybe Gresham or Sandy. Going south, you might even find something as close as Milwaukie, or a little further in Clackamas County south past Oregon City. I've seen real farms in all of these areas recently, and they're all relatively near to mass transit. The MAX train runs east and west, so Gresham or Hillsboro would be good for that. But Portland has good bus service (Tri-Met) throughout Multnomah, Clackamas and Washington Counties.
Another less desirable option is Clark County, Washington, across the Columbia River. I've seen farms in the Battleground and Orchards area, but development is going hog wild there.
Check out
http://www.findafarm.com/ (Oregon's Farm Real Estate Website)