I'm considering using the bowl that comes with Amy Chun's soups. It weighs an ounce and is made of some sort of biodegrable corn plastic called PLA. According to the directions on the package you can microwave it or pour boiling water in it.
If it is truly biodegradable it doesn't appear to me like it will happen any time soon. According to an article on smithsonian.com it will only compost in a specialized facility and requires heat at 140 degrees for 10 days and specialized bacteria. Outside of that it may take up to 1000 years to biodegrade, about the same amount of time as a PET bottle.
Hmmm, Not so very green after all, and PLA can't be recycled at this point, but on the other hand the bowls can take the heat, according to Annie Chun, and aren't toxic.
And they come with a lid ( lid weighs more). The soup is pretty darn good too ( but not at all light weight).