Comment on $V_{ub}$ from Exclusive Semileptonic B and D Decays

The prospects for determining $\mathrm{\mid }{V}_{ub}\mathrm{\mid }$ from exclusive B semileptonic decay are discussed. The double ratio of form factors $(f^{B\to\rho}/f^{B\to K^})/ (f^{D\to\rho}/f^{D\to K^})$ is calculated using chiral perturbation theory. Its deviation from unity due to contributions that are non-analytic in the symmetry breaking parameters is very small. Combining experimental data obtainable from (B->rho l nu), (B->K* l l) and (D->rho l nu) can lead to a model independent determination of $\mathrm{\mid }{V}_{ub}\mathrm{\mid }$ with an uncertainty from theory of about 10%.