Okay, here's what I did for this (and I'm not going to say there isn't an easier way, I just didn't see an obvious tool for doing it).
Load up your picture and make sure the layer it's in isn't locked.
Now click the line tool and hold it down till you get the other options. Select the rounded rectangle tool.
Make sure you've got it on fill mode. You can set the radius of the corner curves on the top tool bar under the Radius option. 10px is default and it's a little harsh. I digress.
Draw the rectangle over the area you want to cut/copy. If you didn't get it just right, you can drag the new shape layer around. If it's to large or small, just delete the layer and do it again.
Once that's drawn, click on the background layer. Then go to the toolbar, hit select -> color range. It should default to the foreground color. If not, simply drop the color selector on the box. so it picks up that color.
Now you've got your rounded rectangle selection box. Now just hide the shape layer, and cut or copy the selection. Make sure you've still got the background layer selected.
Tada!!
Sorry if this is a bit schitzophrenic. I haven't slept in a couple days.