Comment 1 for bug 490987

I believe this comes from the fact that the pointwise call always sets on_boundary to be false:

http://bazaar.launchpad.net/~dolfin-core/dolfin/main/annotate/head%3A/dolfin/fem/DirichletBC.cpp#L653

Also this method for a pinpoint will constrain more of the pressure than you want, for better results do:

class Corner : public SubDomain
  {
  public:
    bool inside(const double* x, bool on_boundary) const
    {
      return (std::abs(x[0] - 1.0) < DOLFIN_EPS ) &&
                (std::abs(x[1] - 1.0) < DOLFIN_EPS);
    }
  };

Of course you have to apply this as pointwise because the geometric and topological searches only look at facets and this is only hitting one point.

-- Andy