Now, with American shows, I think there could easily be a good business model - we've seen English TV channels pick up American shows (sometimes running almost in sync with America, more often than not running a year or two behind) with success. Examples include The West Wing, Friends, Glee, Will and Grace, Family Guy, CSI and so on. So I think there could be enough of an audience over here to be worth doing it.
But iPlayer for non-UK viewers, it just isn't feasible. Firstly, BBC have the content licensed for UK viewing only, and they can't pay extra for international viewers because they are funded by the public (TV license over here). For the BBC website, international viewers see adverts, but that model can't be extended to iPlayer for a few reasons. Firstly, and least importantly, BBC never puts advertising into its shows, and they don't want to go against that, even for an international audience. That could be got around quite easily. Secondly, selling video adverts is a lot more complex (from a sales point of view, not technical) than shoving them into a website, it would take more work from them. And finally, if they were to sell adverts and open up iPlayer to an international audience, I doubt they would make enough revenue from the adverts to justify internationally licensing the content, i.e. there just wouldn't be enough people watching it.