@ Paul, AC. When using Opera Mini, to some sites it sends a generic Opera Mini user agent, to others, it makes a combined Opera Mini/handset specific user agent generated by the proxy. Try it on a couple of those "what's my IP" sites, you'll see. Even when it's just the generic opera mini one though, your device is still being identified in the header seperately fromthe user agent by "X-Opera-Mini-Phone-UA=<original deviceUA" and "OperaMiniPhone = <device name>".
Maybe this is different if you've installed OM yourself rather than getting a phone with it reloaded.