RhysD: Here is my attempt.
The following parenthetical comment is wrong, considering fractic's comments below. (Actually, should be saved as a ko threat; white is already dead.)
fractic: Not so fast, White can play under the stones too.
fractic: White gets a ko like this. The In a Korean baduk school page has a link to the actual solution.
In the second diagram, if black captures the four white stones immediately, he fails; white takes the corner with an under the stones tesuji.
Dave: I think Black has to increase the sacrifice to yield a better shape.