With version 12 Opera finally caught up with Internet Explorer and Firefox in allowing you to set the useragent to whatever value you like instead of just the IE and Firefox alternatives previously offered.
To be able to set your own custom user agent string you need to type opera:config in the address bar and then search for "Custom User-Agent" in the quick find box (Note that the capitals have to be entered exactly as shown or it will not find the entry as the Opera config search facility is unfortunately case sensitive. This will open a input box where you can enter your user-agent string to spoof the settings.
Note that setting the useragent this way does not affect the entries available in the Preferences select list that you can attach to one of the toolbars that allows you to easily change between the Opera, IE, and Firefox default useragents and so if you are going to set a custom useragent then there is no point in displaying that option in a toolbar. The Site Preferences/Network option for Briwser Identification als still restricts you to the standard five entries of earlier Opera versions but would still be useful with sites that stupidly restrict access to those using an IE or Firefox userragent.
This article written by Stephen Chapman, Felgall Pty Ltd.