I already own Wings of Liberty and am hoping that Heart of the Swarm is sold as an expansion instead of a full game. Is there any information about this?
Right now, Amazon.com has Heart of the Swarm listed for $39.99 (USD).
This may just be a pre-order price for it; and subject to change after release. But you can lock into the price now if you pre-order.
If you purchased Wings of Liberty, you still need to purchase Heart of the Swarm in order to play Heart of the Swarm.
According to Blizzard's FAQ for North America and Europe:
How much will Heart of the Swarm cost?
We typically don’t provide details about pricing until the game is closer to release. We do view Heart of the Swarm as an expansion set, so for the regions that have a standard box business model such as North America and Europe, we will price accordingly. For other regions that have alternative business models, we’ll provide details at a later date.
There you have it from Blizzard directly. They consider Heart of the Swarm as an expansion.
To your questions in comments - in the FAQ, you must own Wings of Liberty to play Heart of the Swarm.