Some Netflix titles have Dolby Digital Plus 5.1 sound. I know that TiVos will bitstream that over HDMI but I'm not certain whether they will convert it to basic DD 5.1 for output over S/PDIF or to AVRs which can't decode DD+. Since TiVo has to license Dolby tech they probably do.
There's no filter or even indication on Netflix's site of which titles have DD+ sound; there are some Netflix search sites which do have filters for 5.1, such as TVandMoviesNow.com
(that link has the 5.1 filter on it).