  We compute the $B\to\pi\ell\nu$ semileptonic form factors and update the
determination of the CKM matrix element $|V_{ub}|$. We use the MILC asqtad
ensembles with $N_f=2+1$ sea quarks at four different lattice spacings in the
range $a \approx 0.045$~fm to $0.12$~fm. The lattice form factors are
extrapolated to the continuum limit using SU(2) staggered chiral perturbation
theory in the hard pion limit, followed by an extrapolation in $q^2$ to the
full kinematic range using a functional $z$-parameterization. The extrapolation
is combined with the experimental measurements of the partial branching
fraction to extract $|V_{ub}|$. Our preliminary result is $|V_{ub}|=(3.72\pm
0.14)\times 10^{-3}$, where the error reflects both the lattice and
experimental uncertainties, which are now on par with each other.
