Apple does not allow other browser engines.
The "alternative browsers" in the app store are simply different UI wrappers around the stock Safari.
Yeah. As far as the rumor goes, Opera was denied once already.
Before everyone insists that you don't need another browser, etc, etc, etc, the selling point of Opera Mini is the compression it uses. It works EXTREMELY well, and their claims of 6x speed is certainly true on slower networks (slower than 1MB)