Okay, this is the second time I've fallen over this, and each time I have to search to work out what caused it. So I'm posting here so I can find it more easily, and perhaps help others.
When I am building my app using the command line, I end up with an app that loads but the page is greyed out (grayed out for Americans) and unusable. As though the Load progress option has not completed properly.
The answer is that my theme doesn't have the proper background elements because I didn't update it. Or in this case, I did update it, but I was pointing to the old theme in the command line options.