Quasi-isometric rigidity for the solvable Baumslag-Solitar groups, II
  Let BS(1,n)= < a,b: aba^{-1}=b^n >. We prove that any finitely-generated group quasi-isometric to BS(1,n) is (up to finite groups) isomorphic to BS(1,n). We also show that any uniform group of quasisimilarities of the real line is bilipschitz conjugate to an affine group. 
Benson Farb and Lee Mosher