I'm running Magento Enterprise Edition v1.9.0.0 and have discovered that you cannot use the Full Page Cache with multiple themes controlled by design exceptions (i.e. User-Agent strings).
The cache does not take into account the theme name, and serves up the page with whatever theme it was first cached with. This means that desktop users could see a mobile theme or vice versa.
I discovered there is a fix for EE v1.11
It tries to override a function called _createRequestIds which apparently does not exist on 1.9. I also tried to put the code from the fix into my 1.9 processor, at Enterprise_PageCache_Model_Processor but I couldn't fetch the current theme name for whatever reason.
So -- is there a way to use the FPC on 1.9 with multiple themes / design exceptions?
Or, alternatively, can anyone hack the 1.9 Enterprise_PageCache_Model_Processor to add the current theme name to the URI string before it's cached?