Opera works differently to most web browsers, using a central proxy server. You connect to a central Opera server that caches web pages and delivers them to the app. By caching information and optimising it the Opera web browser can deliver web pages more quickly.
However, it also initially contravened Apple's ban on non Webkit browsers and also there were concerns that it could run external code, also contravening Apple's app store agreement.
Opera expects to launch Opera Mini for the iPad as soon as it has been through the Apple App Store approval process.
Note: this story has been updated after Opera contacted to state they have yet to submit the app to Apple.