I'm now of the opinion that Opera is performing to the standards but is presenting a problem from a poorly written Web app.
This forum's version of vBulletin is fine because it returns a HTTP status of 302 to the "New Posts" request.
The version I use on the other forum returns a 301. It's the combination of a 301 and Cahce-Control: private that means the permanent redirection is cached. Looking for "New Posts" has the effect of resending an old query.
Ho hum.
