Shallow submodule update/clone?

Is there a way to cause 'git submodule update' to perform a shallow  
clone of some or all of a repo's submodules?

Ideally, I'd like to be able to specify that particular submodules be  
shallowly cloned all the time (perhaps by specifying a default --depth  
when doing 'git submodule add ...').  Just an idea, there.

Cheers,

- Chas