We examine the use of the thermal fluctuations of the cantilever of an atomic force microscope to study the microrheological behavior of fluids near a solid/liquid interface. A model-independent approach is used far the analysis of power spectral densities and to extract frequency-dependent dissipative and induced-mass contributions of the fluid to the force experienced by the cantilever. The approach provides a framework for the calibration of AFM cantilevers using thermal fluctuations in viscous fluids and for extracting the loss and storage moduli of viscoelastic fluids. The results show that in viscous fluids the excess zero-frequency viscous dissipation (i.e., relative to the magnitude in the bulk) caused by a nearby surface scales inversely with the distance between the cantilever and the surface, in contrast to the inverse cubic scaling assumed in the literature. Interestingly, the observed scaling is practically identical to what is expected for the increase in the hydrodynamic drag on a sphere descending normally toward a flat surface at low Reynolds numbers.