don't enclose eigen in a namespace. prefixing is the way to go.

For example Qt uses a prefix and no namespace (except for a few things)

src/internal/MatrixRef.h

@@ -26,9 +26,6 @@
 #ifndef EIGEN_MATRIXREF_H
 #define EIGEN_MATRIXREF_H
 
-namespace Eigen
-{
-
 template<typename MatrixType> class MatrixRef
  : public EigenBase<typename MatrixType::Scalar, MatrixRef<MatrixType> >
 {
@@ -60,6 +57,4 @@ template<typename MatrixType> class MatrixRef
     MatrixType& m_matrix;
 };
 
-} // namespace Eigen
-
 #endif // EIGEN_MATRIXREF_H