  We study the superconducting transition in the attractive Hubbard model in
two dimensions using the self-consistent $T$-matrix approximation. We
demonstrate that for large system sizes, this approximate method produces XY
critical scaling in the correlation length and pair susceptibility. For the
parameters we investigate, the critical regime is quite large, extending beyond
5 times $\TKTB$. We discuss the role of vortex-pair unbinding in the normal
state in the context of pseudogap behaviour and the relevance to the physics of
underdoped cuprates.
