  The semileptonic process, B --> \pi l \nu, is studied via full QCD Lattice
simulations. We use unquenched gauge configurations generated by the MILC
collaboration. These include the effect of vacuum polarization from three quark
flavors: the $s$ quark and two very light flavors ($u/d$) of variable mass
allowing extrapolations to the physical chiral limit. We employ Nonrelativistic
QCD to simulate the $b$ quark and a highly improved staggered quark action for
the light sea and valence quarks. We calculate the form factors $f_+(q^2)$ and
$f_0(q^2)$ in the chiral limit for the range 16 GeV$^2 \leq q^2 < q^2_{max}$
and obtain $\int^{q^2_{max}}_{16 GeV^2} [d\Gamma/dq^2] dq^2 / |v_{ub}|^2 =
1.46(35) ps^{-1}$. Combining this with a preliminary average by the Heavy
Flavor Averaging Group (HFAG'05) of recent branching fraction data for
exclusive B semileptonic decays from the BaBar, Belle and CLEO collaborations,
leads to $|V_{ub}| = 4.22(30)(51) \times 10^{-3}$. PLEASE NOTE APPENDIX B with
an ERRATUM, to appear in Physical Review D, to the published version of this
e-print (Phys.Rev.D 73, 074502 (2006)). Results for the form factor $f_+(q^2)$
in the chiral limit have changed significantly. The last two sentences in this
abstract should now read; "We calculate the form factor $f_+(q^2)$ and
$f_0(q^2)$ in the chiral limit for the range 16 Gev$^2 \leq q^2 < q^2_{max}$
and obtain $\int^{q^2_{max}}_{16 GeV^2} [d\Gamma/dq^2] dq^2 / |V_{ub}|^2 =
2.07(57)ps^{-1}$. Combining this with a preliminary average by the Heavy Flavor
Averagibg Group (HFAG'05) of recent branching fraction data for exclusive B
semileptonic decays from the BaBar, Belle and CLEO collaborations, leads to
$|V_{ub}| = 3.55(25)(50) \times 10^{-3}$."
