Show ads in iphone application. Is iAds the only way or there is another alternative?

I want to show ads in my iPhone application. i can do it using iAds. I need more details regarding this.

My first question is: is iAd mandatory if i have to show advertisement in my iphone application? or i am allowed to use some other alternative to display ads.

will iAds work in earlier versions of iPhone ?

can i test iAds outside USA ?

Apple takes 40% when we use iAds, is there any other alternative? so, we don't have to share 40% of the amount every time ?

I hope, i wrote everything in the way its easy to understand.

Thanks in advance.

Random google search turns up at least 10 other ad networks:

1. AdMob
2. Brightroll
3. Google AdSense for Mobile Applications
4. Greystripe
5. InMobi
6. Jumptap
7. MdotM
8. Millennial Media
9. SAY Media [VideoEgg]
10. ZestADZ

and at least one meta ad engine that combines all of the others here.

I have no idea which ones are good, though.