I agree. The trick with PETSc is not only to install it, but to get other software that build upon it detect it correctly (e.g., SLEPc (#334), TAO (#335) and [FEniCS](http://fenicsproject.org)). All play well together with the layout I propose, though I realize this layout is not quite standard. Those other pieces of software typically look for a place called `#{petsc_arch}`.