I too have a cracked top case and Apple Support said that this would be swapped out without problem, just take it to an Apple Store.
However, I also have the issue of cracks in the lower casing. These appear to be stress fractures that run through the screw holes in the lower casing.
There were a number of posts on the internet suggesting that Apple had included these in their warranty coverage, but my experience with Apple Support has been somewhat different.
The person I first spoke with said that she was aware of the issue, but she needed my serial number.
When I called back with my serial number that person wasn't available and the support person who took my call stated that this wasn't covered by warranty.
My call was escalated and I was told again that this wasn't covered and was considered to be accidental damage.
I hate to be told that a manufacturing defect, the stress fractures through nearly every screw hole, has been caused by my actions.
What I would like to know is how these reports found their way on to the internet, how some people are claiming that they have had the lower case replaced and why one person at Apple knew about it but nobody else did!
I am also seeing the same kind of cracks appearing at the edge of the lid near the iSight camera as I am on the top case. The thin sliver of case splitting away.
This is exactly where your thumb or finger would press when you lift the lid of a MacBook, but apparently this is accidental damage as well!