There seems to be an inherent paradox in saying that you need to think about your server-side architecture but you should just be building old-fashioned page by page submissions before hijacking them with Ajax. But the paradox can be resolved if you think about Hijax this way:
* Plan for Ajax from the start.
* Implement Ajax at the end.
If you build your Ajax apps like that, you can have a Web 2.0 app and a Web 1.0 website, as recently tested by Chad Dickerson. He ran two Ajax apps (Gmail and Backpackit) through Lynx, the text-based browser. Gmail fails miserably, Backpackit works just fine.