I know Python apps are faster to write, but it seems Java is the 800 lb gorilla for mobile and  GUI development.
Are there any mobile platforms that run Python, or should I go the Java route?
Java is certainly available on more platforms.  I would pick a target platform (or set of targets) and see what language(s) would require the least number of redundant implementations. 
Also, when you get to a certain level of complexity, the language often doesn't factor into speed.  For initial prototypes, sure some languages are lightning fast to develop in, but by the time you've taken care of all the exception cases and tripled the entire schedule due to QA, you'll find that the prototype development speed wasn't all that big a deal.
Of course, depending on the complexity of your app, this may prove to be untrue--a small simple app can be delivered in near "prototype time".
Python or Java? Whats better for mobile development, and GUI applications