NetFlix would have to re-work their system. Right now they use Flash, and Flash video won't work on the TiVo - ANY of them. The video decoders on-board do not support the On2 codec FLV uses.
Down the road NetFlix is supposedly going to switch to Microsoft Silverlight, which uses WMV. The Series3 and TiVo HD have hardware that can support WMV (VC-1 == WMV9, basically), but it would remain to be seen if the streaming format for Silverlight could be made to work. And that still blocks the vast majority of TiVos - to the point that it probably isn't worth it for Netflix. The S2/S2DT are the vast majority of units, and it will be a while before the HD is a significant market on its ow (the original S3 will remain negligible, but can be lumped in with the HD).