Actually, the Castle show isn't based on a Marvel comic, however Marvel did publish a tie-in comic for the show. But like I said, Marvel didn't create the character or anything, they just published a licensed spin-off, kind of like how Dark Horse publishes Star Wars comics (even though the movies came out long before DH ever started their comic run of SW titles...which ends next year when Marvel starts publishing the new SW comic titles).
And Smallville was definitely based on DC's Superman, but The Walking Dead is actually a title published by Image Comics.
But yeah, Marvel hasn't really had a presence in the world of live action TV since The Incredible Hulk series of the late 70's/early 80's. Well, you could say Mutant X and Blade - The Series qualify, but those are probably best left forgotten, especially since Mutant X wasn't even based on Marvel comics characters, but just kind of borrowed the premise of mutants with powers from the X-Men (without actually using the X-Men or related characters so they wouldn't have to pay Marvel to use their characters).
As for the future, we'll see. Obviously Agents of SHIELD has a lot of hype going for it, so the showrunners would seriously have to screw it up to get taken off the air (kind of like what happened with Heroes). And if it's successful, I can definitely see Marvel wanting to try shows based on other characters and properties they own. There's an Incredible Hulk tv show supposedly still in the works, but that's been rumored for years and I honestly think it's been scrapped in favor of the movie version of Hulk (Marvel probably doesn't want to confuse consumers or water down the brand by having both a movie version and live action TV version going on at the same time). Animation is a different story since in large part that's still thought of as being aimed strictly at children.
But yeah, with Disney's acquisition of Marvel, and if the SHIELD show does well, I can see the Disney execs trying out other Marvel properties in various forms on various channels. Probably more animated shows on Disney or Disney XD, perhaps a lower budget live action drama/comedy on ABC Family, etc. I think the SHIELD show will be the only one connected to the MCU, at least for the time being.