Another option is to have all necessary information about the current operation in the URL. This is highly scalable, since you don't need to keep track of anything user-specific on the server(s), and the navigation supports branching and back/undo just like you describe.
Strangely enough PG specifically disallow this approach in his competition!
Most web apps need _both_ global session state and URL-based state. As others have pointed out, if you browse a product catalog, you would like to be able to branch into different browser windows or use the back-button. However, when you add an item to the shopping basket, you want it to be a global state change (you want have the same shopping basket in all windows), and you don't want a buy to be undone by clicking back.
Continuations are only an options for handling URL-based state, not for handling global state. And for page state they have some limitations.
For example, if all navigation is handled by continuations, you basically have to store a continuation for every hit indefinitely, since you dont know if the user have bookmarked the URL. If you don't want to store the continuations forever, you should only use them on pages that are not bookmarkable anyway, i.e. pages that are the response to form posts. But then the stated advantages, like the ability to branch and use the back button is moot, since you cannot do that anyway with form responses.
Continuations are really nifty for quick prototypes of web apps, but for production use, I believe they are a leaky abstraction.
