I don't have any stats for Cornell, but I do have some info about other schools.
For Harvard, I have heard that they admit about 50-60% of those that they interview.
For Stanford, an adcom actually said that they hope interview 800 students and admit 400-500 this year (for 370 spaces).
I believe that Wharton also admits about 50% of those that they interview.
These numbers are off the top of my head, but I believe they are more or less accurate. Getting an interview definitely doesn't mean you are a sure thing, but I think it moves you from 10-20% chance to 50%+ chance.
Of course, some schools interview everyone that applies so their admit rate for interviewed candidates should be the same as their overall rate, and at quite a few schools less than 1% of students are admitted without an interview.