The "Viewport Alpha" settings do not seem to work for the Glass shader the way it works for the Transparent shader (or a mix between Transparent and another shader). However, if you set the Viewport Alpha to "Add" you will get a sort of fake viewport transparency, which looks about the same for any shader. Maybe that's good enough for your needs?
Otherwise maybe keep a couple of different shader setups in your material. Use a simple mix of transparent and glossy (or diffuse, or whatever) for your viewport editing needs. When it comes time to do a "real" render, switch the inputs to your glass shader.
Edit: here is how these options look in the viewport ("Material" view), as suggested by @LukeD:
The cube and suzanne to the left have the same material. It is a glass shader, with Viewport Alpha set to "Add". The cube and suzanne to the right share a different material. It is a 50/50 mix of transparent and glossy shaders, with Viewport Alpha set to "Alpha Blend".