Hmm ... maybe something like this would help?:
HEXUS.net - Headline :: Compro VideoMate S350/300 satellite TV tuner cards : Page - 1/1
The only problem being is if your actually paying for your sat' service then it will have it's own decrypter. The above and it's like is only for 'free' satellite services. I'm not sure what it's like in the states but here in blighty there is plenty of 'free' tv to warrent not paying for a service (although you do still have to pay for a yearly licence). I guess stuff like HBO you have to pay for ( I once asked a friend who lived in New England if they'd ever watched Sex and the City, they replied that as they couldn't afford HBO that the answer was no.) Anyway, I digress...
I think your idea of putting a line in from your original sat reciever to your media center is going to be your only option if the above won't do whats needed. As you say, your going to have box pretty clever with remotes ect..
Sometimes though once you've built a thing, you can then expand it when and if the technology becomes available. Or you just may find the solution in the build process as sometimes these process take you to situations you weren't in before... Sorry if thats coming across a little muddled but hopefully you'll get my drift.