By using the Ginzburg-Landau theory we studied the order parameter nucleation in the planar superconductor-ferromagnet (S/F) hybrids of a finite thickness, when the magnetic field is inhomogeneous both in the lateral and transverse directions. We calculated the dependence of the critical temperature T-c on the external magnetic field H and studied the transformation of the phase transition line T-c(H), induced by the variation of the parameters of the S/F bilayers such as the period of the domain structure 2L and the thicknesses D-s and D-F of the superconducting and ferromagnetic films. The actual shape of the T-c(H) line is demonstrated to be determined by the nucleating regime at H=0. For the thin-film S/F hybrids (L/D-F > 1) a possible type of the phase boundary T-c(H) is predicted-the reentrant superconductivity with the positive slope dT(c)/d parallel to H parallel to at H=0, which appears as a consequence of the order parameter nucleation at the centers of the magnetic domains at H=0. The nucleation near the domain walls at H=0, resulting in the T-c(H) line with zero slope dT(c)/dH parallel to(H=0), can be stimulated, e.g., by increasing the D-s value or decreasing the ratio L/D-F.