KISS as they would say. Keep It Simple and Stupid
First of all, the data source :
you have to have contributors, whose contribution is easy to fetch, so database backend it is, on a collaborative solution.
Therefore, relying on the Joomla ratings tables fed by the WW review applet is, IMHO the only way to go.
The other way would be to copy the reviews in your own base, but you're facing evolution (no "dynamic" content) and rights issues, as you so clearly explain.
To be technology agnostic, besides, to cover not only iPhone, but also Android, and maybe the last versions of blackberries, would be to go "web app", once again, IMHO.
I propose, (and that's what I'll do) using a mobile templating framework (such as iWebKit), and implement the features, PHP based, hosted on the WW server, and exploiting the WW review application's database.
As far as being "site agnostic" is concerned, in the end, any joomla-based absinthe-oriented web site could have its own version of the app.
Possibly then, WebService exposure, and, with rights provided, gathering of data...
Also, Ron, technically, it would be feasible to have something independent, using a crawler and a lexer, but hell, that would be horribly difficult to implement