  We have measured the irreversible magnetization of an overdoped
Bi$_2$Sr$_2$CuO$_6$ single crystal up to B=28 T and down to T=60 mK, and
extracted the irreversibility line $B_{\rm irr}(T)$: the data can be
interpreted in the whole temperature range as a 3D-anisotropic vortex lattice
melting line with Lindemann number $c_{\rm L}=0.13$. We also briefly discuss
the applicability of alternative models such as 2D- and quantum melting, and
the connection with magnetoresistance experiments.
