  Recent experimental results on $B\to\rho\rho$ decays indicate that the CP
asymmetry $S_{\rho^+\rho^-}$ will give an interesting determination of
$\alpha=\arg[-(V_{td}V_{tb}^*)/(V_{ud}V_{ub}^*)]$. In the limit when the $\rho$
width is neglected, the $B\to\pi\pi$ isospin analysis can also be applied to
$B\to\rho\rho$, once an angular analysis is used to separate transversity
modes. The present bound on the shift of $S_{\rho^+\rho^-}$ from the true
$\sin2\alpha$ is already stronger than it is for $S_{\pi^+\pi^-}$. We point out
a subtle violation of the isospin relations when the two $\rho$ mesons are
observed with different invariant masses, and how to constrain this effect
experimentally.
