While studying how iPhone and Android display PC websites on mobile phones (here and here) , I came to the conclusion that physical screen size doesn’t really matter that much; auto-resizing text based on complex software algorithms makes all the difference.
Since there is a large difference in how browsers auto-resize text, it would be unwise to rely on it for a conference system where mobile access is very important. Ideally, you should create mobile versions of all the relevant websites for the conference. If you do not have the resources for this, at least make sure that the websites look good on Android devices, because PC websites are more difficult to use on the default Android browsers. Checking with mobile Safari is insufficient because its software algorithms make up for your mistakes.
Looking into the future, FireFox and especially FireFox OS may change the situation. The current versions of FireFox on Android use auto-resizing very aggressively, and we can expect the low-end phones using FireFox OS (which will use small screens to cut costs) to do the same. These phones may make browsing PC websites on small screens quite bearable.
If auto-resizing becomes commonplace and web designers can begin to rely on it, then the extra efforts to optimize websites for mobile will become less important. A single design will cover both PCs and smartphones.