Yea, we were hitting visible load issues on both the images and jQuery. Because the goal was to build something that felt like an iOS app, the longer the load screen was up, the less it felt like a real app.

The em technique is discussed in some detail here: http://clagnut.com/blog/348/

In practice what it also allows us to do is to set a font-size of 150% on the body and everything scales up proportionately.