With APDE, it is now possible to write sketches on your phone / tablet without a computer!
From the Android Processing Wiki:
We currently have no plans to make a version of the development environment (the PDE) that runs on the phone, but that might be fun for you internetpeople to hack together on your own, and amaze us all.
This is no longer the case.
The source code is available on GitHub. A detailed description of the app and development for the app is available in the README file on GitHub.
If you have any questions, ideas, bug reports, or comments, you can leave them below.