As a parent, I will tell you that if my son did this to me; that iPad would be the last thing I ever bought him. Appreicate what you already have!
My son wants an iPhone. And I will get him one........right after he brings me a 3.5 gpa on his first quarter report card. Doesn't get a 3.5 gpa, doesn't get an iPhone. Even tells a small lie to me about this topic.....doesn't get an iPhone.
Truthfully, I could care less if my son doesn't have a 3.5 gpa on his report card. All I care about is that he is HONEST, and works HARD. I know my son well enough to know that he'll get that 3.5.
I think you need to reevaluate what your parents have done for you in your life.......
Want an iPhone 4, sell your 3gs. Take that $, and buy an iPhone 4. Selling the 3gs doesn't cover the amount you'll need..........simple, sell more of your belongings (which your parents paid for anyway).