  A dense vortex lattice in a rotating dilute Bose-Einstein condensate is
studied with the Thomas-Fermi approximation. The upper critical angular
velocity Omega_{c2} occurs when the intervortex separation b becomes comparable
with the vortex core radius xi. For a radial harmonic trap, the loss of
confinement as Omega approaches omega_perp implies a singular behavior. In
contrast, an additional radial quartic potential provides a simple model for
which Omega_{c2} is readily determined. Unlike the case of a type-II
superconductor at fixed temperature, the onset of Omega_{c2} arises not only
from decreasing b but also from increasing xi caused by the vanishing of the
chemical potential as Omega approaches Omega_{c2}.
