I realize that if I choose to distribute a custom module for Drupal that I must provide equal access to the source code of said module (without obfuscation). My question is what about a phone app that used data from a Drupal Module? Do I have to release the code to the phone app as well?
The app would not function entirely without the data, which it would receive with either a standard get or post request to a url defined by the module.
If you use a GPL'd code, you have to make your code GPL, if you use a GPL'd code output, you don't have to, it's the same if you use gcc to compile your code, the output doesn't have to be GPL'd.
All the code that depends on Drupal has to be GPL. So a phone app that uses data from Drupal is not dependent on Drupal. Any system could provide the data.
In a Drupal theme, js, CSS and templates could be lisences under whatever sinse they could be used by anything. But all the stuff in template.php would have to be GPL since it's code that work only with Drupal.
Stack Overflow is probably not the right place for legal advice, consider any answers here as guesswork, not reliable legal advice.
Nevertheless, the important part here is if your app is a derived work of the GPLed Drupal. As far as I understand it, a Drupal module would be considered a derived work and therefore be subject to all GPL restrictions. If you use a general interface (scraping HTML, some kind of web service) your application would not be a derived work and you would not need to abide by the GPL rules.
My personal guess would be, as long as you don't call any function of the Drupal API, that you're safe.
One sentence from the wikipedia (another unreliable source on legal questions):
The mere act of communicating with other programs does not, by itself, require all software to be GPL; nor does distributing GPL software with non-GPL software.