  One of the most remarkable properties of the high-temperature superconductors
is a pseudogap regime appearing in the underdoped cuprates above the
superconducting transition temperature T_c. The pseudogap continously develops
out of the superconducting gap. In this paper, we demonstrate by means of a
detailed comparison between theory and experiment that the characteristic
change of quasiparticle dispersion in crossing T_c in the underdoped cuprates
can be understood as being due to phase fluctuations of the superconducting
order parameter. In particular, we show that within a phase fluctuation model
the characteristic back-turning BCS bands disappear above T_c whereas the gap
remains open. Furthermore, the pseudogap rather has a U-shape instead of the
characteristic V-shape of a d_{x^2-y^2}-wave pairing symmetry and starts
closing from the nodal k=(pi/2,pi/2) directions, whereas it rather fills in at
the anti-nodal k=(pi,0) regions, yielding further support to the phase
fluctuation scenario.
