  The prospects for determining $|V_{ub}|$ 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 $|V_{ub}|$ with an uncertainty from theory
of about 10%.
