  We report measurements of partial branching fractions for inclusive charmless
semileptonic B decays Bbar --> Xu l nubar, and the determination of the CKM
matrix element |V_{ub}|. The analysis is based on a sample of 467 million
Upsilon(4S) --> BBar decays recorded with the BaBar detector at the PEP-II e^+
e^- storage rings. We select events in which the decay of one of the B mesons
is fully reconstructed and an electron or a muon signals the semileptonic decay
of the other B meson. We measure partial branching fractions DeltaB in several
restricted regions of phase space and determine the CKM element |V_{ub}| based
on four different QCD predictions. For decays with a charged lepton momentum
p_l^* > 1.0 GeV in the B meson rest frame, we obtain DeltaB = (1.80 \pm 0.13
(stat.) \pm 0.15 (sys.) \pm 0.02 (theo.)) \times 10^{-3} from a fit to the
two-dimensional mX-q^2 distribution. Here, mX refers to the invariant mass of
the final state hadron X and q^2 is the invariant mass squared of the charged
lepton and neutrino. From this measurement we extract |V_{ub}| = (4.33\pm 0.24
(exp.) \pm 0.15 (theo.)) \times 10^{-3} as the arithmetic average of four
results obtained from four different QCD predictions of the partial rate. We
separately determine partial branching fractions for B^0 and B^- decays and
derive a limit on the isospin breaking in Bbar --> Xu l nubar decays.
