I guess that we will just have to wait and see. If has not been verified if Voldemort knows, or doesn't know, just what is going on with Snape. We can speculate... but you don't know for a fact that he knows Snape has abandoned him. I would love to see him try to go after Snape though, and I would bet money that Dumbledore will be protecting him.
:::>^..^<::: ~*~The Journey is more important than the end or the start~*~ :::>^..^<:::
I believe Voldemort considers Snape his spy at Hogwarts. So, that bought Snape time to go and appear before him. We do know that in the Goblet of Fire Snape left, and he was spying for the Order in this last book. I am sure that Snape wormed his way out of his late appearance. Plus, Voldemort isn't exactly patient, so I am sure that is where the "coward" comment came from.
Maybe Snape isn't dead because that's what Voldemort wants. Snape suits his plan for the time being, and he is letting him live. Or maybe, Voldemort doesn't want Snape to know that he knows that Snape has left him!
:::>^..^<::: ~*~The Journey is more important than the end or the start~*~ :::>^..^<:::
I don't think he has an Imperius Curse on him. People under it seem a bit aloof and in their own heads. Snape is in control of himself and very aware of his surroundings.
"You're disturbing...yet intriguing. Like couscous. I'm gonna call you Couscous from now on."
snape could have blamed his late appeance on the fact he cant apparate from Hogwarts, and i think Karkaroff person who abbondoned him (who he will kill) and snape was the coward, (who he could punish)