This paper demonstrates the application of the Finite-Difference Time-Domain method for dispersive media to indoor ultra-wideband channel modeling. A new description of the frequency dispersion of building materials, based on a partial-fraction approach, is proposed, utilizing experimentally measured data on complex permittivity values reported in the literature. The analytical dispersion model for a series of building materials is estimated through the Vector Fitting technique and the through-the-wall penetration is calculated for indicative cases. Finally, a small two-dimensional office environment is studied and several channel characteristics are calculated demonstrating the flexibility and robustness of the proposed formulation in communication modeling. The proposed FDTD implementation covers all the bandwidth in a single run instead of running simulations for every frequency or subband.