How can you make the mesh in a 3D view a bit transparent? I'm modelling with a reference image as a background image, but I can't see it through the mesh. How can I make it a bit transparent in the 3D view so I can see the image?

NOTE: I'm doing this in Cycles.
You want to set your background image like this:



Set it to front and give it some opacity.

Transparent mesh you can only make in Blender Internal in GLSL shaded viewport by setting the material Z Transparency Alpha or by giving it a transparent texture.