See, the problem with a movie like Burn After Reading is that people are expecting it to be something it really isn't.
It's only funny if you're the sort of person that thought Layer Cake or In Bruges was funny. The it's not comedy so much as just really morbid fantasy that's not-quite-ironic.
I thought it was a rather brilliant movie about how people will continue to dig the hole a little deeper for themselves and everyone around them because they keep saying "Dig up, stupid!"... And at the end of it, we learnt not to do it again, whatever it was in the first place.
But we never learn what it is in the first place.
The way I see it, the whole movie rests on you understanding that last line as being something you're meant to realise is happening in your own life.
Also, the bit where they finally wipe the smile of Brad Pitt's face is great!