I am learning Google App Engine / Python and I love it. Unfortunately I am not allowed to use my own computer in the office and not allowed to install anything on the corporate machine.
It would be so great to have an online IDE for Google App Engine where I could play with my Python code using any browser, including the one on my iPad.
Maybe http://www.devunity.com/ will be an online IDE for GAE some day. Do you know any existing way to program GAE in a browser?
Py I/O: not quite a full IDE, but more like a console where you can try out some of your Python code. And it is written and running on GAE.
I don't think there is a full online IDE out there at the moment that will allow you to write a full GAE/Python project.
One option is to use web2py. All of the files in a web2py project can be edited by the framework's own web-based editor. A fairly good overview of how this works, and some of the features of the site is available on this video:
For now and probably for quite some time you won't be able to edit your app directly on an app engine account (as @Wobble notes: the file system on the instances are indeed read-only, for good reasons that hardly will change). Things like Py I/O or similar are also to limited, as you note yourself.
To get forward you minimally want a file system that you can write to, preferably a machine that also will run the development server for you. Rent space on some VPS provider (e.g. gandi.net, http://www.gandi.net/hosting/), then you can mount your VPS home directory onto an office machine and create your app at the VPS, then deploy from it. It certainly won't be an IDE, you need to use (/learn) how to handle the system mainly through the command line, but it will be something. And as an added bonus, you could try non GAE python stuff on it (Turbo Gears, twisted stuff, your choice of fancy database etc)