What happens to dextrorotatory amino acids in humans?
For the most part they are not used.  there are amino acid racemases, which interconvert L- and D- forms of some specific amino acids, which may be used in some particular biosynthetic or metabolic pathways.  

In particular I'm thinking of firefly luciferase which uses D-Cysteine as a re-dox reagent to regenerate the luciferin substrate that the light - generating enzyme consumes.  see slide 4 here.  

Nonribosomal peptide synthases often use D amino acids too.  

I don't believe that d- amino acids are really incorporated into proteins though.