B&H Photo now has the 20mm Pancake E-mount Lens in stock now. This is a great little walk around/all purpose lens for the Sony Nex line measuring in ~20mm in thickness. It is a 35mm equivalent in 35mm format with a pretty fast aperture at f/2.8.
Another option is the Sigma 30mm F2.8 EX DN- Sony E, its not too much bigger and the price point is a little lighter.
We hope to our hands on these in the future to do a side by side comparison. Till then check them out Sigma here and Sony here.