Import .obj with vertices' UV specified into Cycles? I have (1) a .obj file in which I specified the UV coordinates for each vertex, (2) a .mtl file pointing to a diffuse texture map, and (3) the said texture map.

When I import the .obj into Blender Internal, the object is correctly and automatically textured. However, when I import it into Cycles, there is no texture. This post seems to suggest I need to manually set the texture, which is actually fine for me.

What confuses me is, if I am to manually set the texture, how can I use the UV already specified in the .obj file instead of doing another UV unwrapping?
When you imported the .obj file, it should have also imported the UV Map that was stored within it. You can verify this under the mesh tab of the properties panel. With the object selected, go to the properties panel (bottom right by default) and click on the mesh tab (looks like a triangle made up of three dots joined by lines).

If you scroll down to UV Maps, you can see all UV Maps applicable to your object. If you have just imported your object and done nothing else, you should see one entry, being the UV Map that you imported along with your object.

Cycles will use that UV Map by default for any image texture in any material that you assign to the object. If you have more than one UV Map on an object you can select the relevant UV Map using the UV Map node, but you should not need to do this unless you have created a second UV Map.