I am experiencing something similar with R2 CU1 - the permissions defined under the SSRS report are honoured in my development environment as Brandon suggests, but when the model is moved to a different environment and SSRS reports are redeployed, the report permissions no longer seem to be inferred.
I have to explicitly add Invoke access on the processReport method (and any other required tables) to the associated privilege to make this work consistently (even though it works in my development environment when launching ax32 as a different user.)