Much of the logic will be client side definitely, but less than I first thought.
One of the problems with this kind of question - which many of us share - is that there will be many ways to build an app, although in time I expect that to settle down into a smaller number of options around several common architecture/design patterns.
Here are my first thoughts for http://safepress.io:
However, since then I’ve stopped work on the backend side of things (which were almost all in the client as you can see above), to await more details of Structured Data which I understand will allow some level of querying or enumeration (but don’t quote me on that because I really don’t understand what is planned).
There are also questions about how dynamic web apps could be implemented. I submitted an RFC on this but the jury is out because I think MaidSafe may have other (probably better) ideas, and they are too busy with other stuff for the time being.
One of the devs, who I shall not name for fear of incriminating them, did say he expects them to begin clarifying some of these issues in a couple of sprints, which suggests before the end of the year (being cautious).