They also mention compatibility with other DVR products, with additional support coming in the future:
The My DVR Expander external hard drive, a TiVo Verified™ product, has been tested for compatibility with TiVo HD and TiVo Series3 HD DVRs. It is certified compatible with Scientific Atlanta 8300 Series digital video recorders (including models 8300 HD, 8300 MR and 8300 HD MR). Further compatibility of the My DVR Expander drive is planned as other cable and satellite DVR manufacturers enable the eSATA (external SATA) ports on their currently-deployed DVRs.
The unit adds a simple 500GB onto the TiVo, providing enough capacity for an additional 300 hours of video, or 60 hours of HDTV recording. The unit is higher in price than what a 500GB drive runs for standalone, at $199, but for an ease-of-use product to simply plug into a TiVo it makes sense. You can read the press release at Western Digital's site.