It already exists: Safari.
That aside, it would be nice to have more smartphone-centric apps specifically for FR, for example to provide alerts or badges on pings, FR mail, and/or state message boards. Other than that, the only other smartphone-based improvement might be layout to make it easier to select links. (Basically, a "mobile web" version of the site, which could be implemented with or without an app.)
Winner! Mobile website is the way to go IMHO.
Agree, a mobile friendly CSS would be helpful. But an app, could have some benefits. For instance: doing HTML tags (just more of the basic tags like, italicize, page break, bold, block quote and image posting) on a mobile device is a bit of a pain. Not impossible mind you, but it really slows things down to switch through all the keyboards to find the special characters you need.
So an app that could help in that regard would be great. Don’t know if such functionality could be part of the CSS.