  The angles of the CKM unitarity triangle are now known well enough to allow a
determination of its sides from global fits, with good accuracy. Assuming that
new physics does not affect the angles, UTfit and CKMfitter find $|V_{ub}| =
(3.50\pm 0.18)\times 10^{-3}$. Using this result, and new high-precision data
on the spectrum of $B\to\pi \ell\nu$ decays from BaBar, we find $|V_{ub}|
f_+(0) = (9.1\pm 0.7)\times 10^{-4}$ and $f_+(0) = 0.26\pm 0.02$, with $f_+$
the $B\to\pi$ weak transition form factor. These results are {\em completely
model-independent}. We compare them to theoretical calculations from lattice
and QCD sum rules on the light-cone.
