According to Creative, only motherboards with Nvidia nForce4 chipsets exhibit the problems. Allegedly, the PCI bus on those chipsets holds off bus master memory requests for abnormally long periods of time. Since the X-Fi's buffers aren't designed to compensate for this, sound coming from the card ends up crackling and popping. Creative claims nForce4 motherboards are the only ones to exhibit this behavior, although the company points out that not all are affected. The nForce 590 chipset is apparently unaffected, as well.
Nvidia, meanwhile, seems unaware of any problems. Nvidia Technical Marketing Manager Sean Cleveland said in an online chat that, "Currently, there are no open bugs regarding Creative’s X-Fi audio card." Thanks to NGOHQ for the tip.