I love your earrings! They really turned out well, and people love the idea of reusing anything nowadays, I bet they'd be a hit amongst your friends and stuff.
I had an idea for the bending issue- I just started making some earrings myself because I have tons of toilet paper rolls lying around, and as I was trying to smooth out that stubborn bend, I thought, why not embrace the bend??? So instead of making "circle" earrings, I decided I'm going to make oval earrings! It certainly holds better as a oval than I circle, I find.
Here's some pictures so you can see:
The side view:http://i48.tinypic.com/2l0qpu.jpg
Straight on view:http://i47.tinypic.com/51kd4.jpg
Also, I was thinking that if you wanted to keep it circle or even enforce the oval, you could just use some sort of strong wire on the inside of the cardboard. Not sure how you would attach it, maybe some glue or something. Just an idea!