No game is ever completely canned. When you put that much work into something (the point where you can show it to someone), you're going to keep it around for... memory sake. Also because you tend to come up with innovative things you'll want the code or design for later, so you keep it as a reference.
The SC: Ghost project may be dead, but it's not gone. It'll hang around, in our minds and probably in some vault at Blizzard. You might be right, they might still be working on it, but judging from the fact that it's not mentioned much of anywhere anymore it probably means they've stopped work on it for now.
Think about it this way, you guys, for D3;
Why would Blizzard keep it secret? Imagine how much hits on their webpage and interest in their products they'd get from something as sought-after as this. Game companies don't have any reason to keep the fact that they're working on a new, amazing game completely secret. Letting out little bits about it keeps interest.
There's still a possibility they're doing work on it, just nothing major at the moment judging by the lack of news about it. It seems right now they're mostly focusing on maintaining the games they have out, and working on World of Warcraft.
But hey, keep your hopes up! It'll probably come out some day, just probably not this year unless it magically shows up at E3.