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/Minor.h

@@ -26,8 +26,6 @@
 #ifndef EIGEN_MINOR_H
 #define EIGEN_MINOR_H
 
-namespace Eigen {
-
 template<typename MatrixType> class MatrixMinor
   : public EigenBase<typename MatrixType::Scalar, MatrixMinor<MatrixType> >
 {
@@ -79,6 +77,4 @@ EigenBase<Scalar, Derived>::minor(int row, int col)
   return MatrixMinor<EigenBase>(ref(), row, col);
 }
 
-} // namespace Eigen
-
 #endif // EIGEN_MINOR_H