For Opera display of Adsense ads on handheld devices may depend on the styling of the site. If your sites does not have a stylesheet for handheld media, Opera will use its Small Screen Rendering technology (SSR). In SSR mode iframes are basically removed.
However, if you have a stylesheet for handheld media, Opera will show the page as styled by you, including iframes and Adsense ads.
You can try this for yourself by running the desktop edition of Opera. SSR technology is included in this version of the browser and can be activated by pressing Shift+F11, or by selecting View->Small Screen from the menus.