I just got off the phone with TiVoPony. It turns out that the S3 will not support Multi-Stream cards yet. Well, it will, but only in single-stream mode, so you still need two cards. Turns out there is no Cable Labs certification for CableCARD 1.0 Multi-Stream yet, just CC2.0 Multi-Stream. They’re working on it. This means page 102 of the Viewer’s Guide, which says you can use one M-Card, is incorrect. (I told Pony about that on the call.)
So, just correcting my earlier statements. Sorry for the misinformation. Not that M-Cards are really available yet anyway. I’ll be updating the S3 FAQ too.