Yup, if you've got a DirecTiVo, what you really have is a DirecTV box with licensed TiVo software on it. TiVo, of course, would be happy to license additional software to DirecTV, but it's up to them if they want to expose their users to it. Given that the NetFlix deal directly conflicts with DirecTV's pay-per-view services, I strongly doubt you'll see any support for this on DirecTiVo gear. This further sucks because the new HD-TiVo would be the ultimate platform upon which to build this. They could use spare satellite bandwidth to transmit the DVD bits, getting you much faster downloads than you can get over DSL or cable modems.
Of course, just like TiVo could make an alliance with Netflix, they could just as easily offer a similar feature with DirecTV, without Netflix directly involved. Hmm.