Experimental Limits on Weak Annihilation Contributions to b -->u ell nu Decay

We present the first experimental limits on high-q^2 contributions to charmless semileptonic B decays of the form expected from the Weak Annihilation (WA) decay mechanism. Such contributions could bias determinations of |Vub| from inclusive measurements of B-> X_u lnu. Using a wide range of models based on available theoretical input we set a limit of (Gamma_WA/Gamma_b->u) < 7.4% (90% confidence level) on the WA fraction, and assess the impact on previous inclusive determinations of |Vub|.