I agree that adding a Portrait mode should be an option. No reason to limit the user experience there IMO.

However, I think that for the most part the better option would be bringing most websites out of the imaginary Land of Unlimited Real (screen) Estate and reducing their layouts to tablet-friendly formats as much as practical and possible. Do commercial websites really need to be wider than 800 pixels? Do they really need to contain over 200 objects per page? Or can more designers get creative with the user experience they present, like I'm doing with www.jablet.net? Sure, I still have a lot of work to do (I'm still learning and experimenting) but I think both tablets and PCs can benefit from a return to basics.