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/RowAndCol.h

@@ -26,8 +26,6 @@
 #ifndef EIGEN_ROWANDCOL_H
 #define EIGEN_ROWANDCOL_H
 
-namespace Eigen {
-
 template<typename MatrixType> class MatrixRow
   : public EigenBase<typename MatrixType::Scalar, MatrixRow<MatrixType> >
 {
@@ -143,6 +141,4 @@ EigenBase<Scalar, Derived>::col(int i)
   return MatrixCol<EigenBase>(ref(), i);
 }
 
-} // namespace Eigen
-
 #endif // EIGEN_ROWANDCOL_H