Ooooops.
A search does find a variety of methods to check camber at home:
https://www.google.com/search?q=check+camber+at+home
where some of those articles used mobile phone apps

I had forgotten to link to a descriptive photo of the desired task:
http://i.cubeupload.com/6CPUl7.jpg

I'm sure there are gotchas (e.g., is the garage floor really flat?), but it
seems doable to measure camber at home if we can answer the main obvious
questions which are (I think):

Q: What accuracy is *needed* to measure camber at home?
Q: What accuracy can be *attained* with a typical mobile device?
Q: Is the repeatability sufficient in a typical home measurement setup?
Q: How do we compensate for typical errors (e.g., ride height, flat floor)?

What other gotchas will we need to look at to successfully measure wheel
camber using a mobile device in a typical garage setup?