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/MatrixStorage.h

@@ -26,8 +26,6 @@
 #ifndef EIGEN_MATRIXSTORAGE_H
 #define EIGEN_MATRIXSTORAGE_H
 
-namespace Eigen {
-
 template<typename Scalar,
          int      RowsAtCompileTime,
          int      ColsAtCompileTime>
@@ -125,6 +123,4 @@ class MatrixStorage<Scalar, DynamicSize, DynamicSize>
     { delete[] m_array; }
 };
 
-} // namespace Eigen
-
 #endif // EIGEN_MATRIXSTORAGE_H