Today I played again with the Google App Engine (Java preview release) for a couple of hours and found another bug.

Seems like the MemcacheService namespaces are completely ignored in production.

Bug 1364 already reported.

The JCache API I used last week was just an higher-level interface built on top of the MemcacheService.
I found that using the low-level API directly is a better approach because it exposes some additional functionalities with no drawbacks.
Atomic-increment of cached entries for example is an extremely useful feature in a clustered environment.