Several recent experimental studies report anomalously large effective thermal conductivities of dispersions of metallic nanoparticles, or "nanofluids." These data are described as anomalous because the level of thermal conductivity enhancement is significantly larger than that predicted by effective medium theory. It has also been reported that the level of thermal conductivity enhancement increases significantly with increasing temperature. This enhancement of thermal conductivity, whose mechanism is not well understood, offers many new and promising applications for this class of fluids. At present, there exists a modest database of thermal conductivity measurements on nano-fluids. Much of these data have been obtained using the transient hot-wire technique. More recent experimental studies employing optical techniques to measure thermal conductivity in nanofluids have yielded results that are consistent with effective medium theory predictions. In this study, we present evidence that the anomalously large effective thermal conductivities obtained using the transient hot wire method may be an experimental artifact of that particular method.