Reflected depth of field

How do you deal with depth of field effect in reflections? That's a good question an probably a pretty difficult problem to solve when starting from scratch (which I enjoy doing... waste of time??). Anyway, here is the sample picture:

 

It's obviously not a computer generated picture, but something exported from my camera :) The postcard is stuck to the mirror and is in the sharp area. So far so good. But the reflection in the mirror is blur! So how do you deal with that in ray-tracing? Maybe raycasting could help? Something to think about.

Any ideas?
Cyrille