I've deployed a -testnet version of ClearCoin, at: https://testnet.clearcoin.appspot.com/
It is fully functional, so feel free to creates some escrow transactions and get some testnet bitcoins from the the testnet faucet (which I will eventually move to testnet.freebitcoins.appspost.com).
For anybody else developing on App Engine, here's what I did to make it work:
In my main.py:
# Set testnet namespace
if 'test' in os.environ['CURRENT_VERSION_ID']:
CURRENT_VERSION_ID is the version of your app that's running, and is set by App Engine.
set_namespace makes all subsequent datastore and memcache operations read/write from a different database. So almost all of the code for ClearCoin doesn't care whether it is handling testnet coins, it just works.
The only other change needed was a check for 'test' in os.environ['CURRENT_VERSION_ID'] when deciding which bitcoind server to use. I run the -testnet bitcoind for ClearCoin on a different machine than the production bitcoind, so experiments on the test net won't affect the production ClearCoin at all.
If you're not running on App Engine, you should think about building in the equivalent of CURRENT_VERSION_ID and 'set_namespace' so deploying test and production versions of your application is easy.