update from 7.12 to 7.14 DrupalDefaultEntityController->cacheGet()

That part of the code has not been changed between these two versions. Did you update/add any other modules while updating? Changing to a support request until we have confirmation that this is actually a bug.

Try to figure out where this is coming from. One way to do that is enable devel.module and add the following code on the line above. Then check who is calling the function with invalid arguments.

foreach ($ids as $id) {
  if (!is_string($id) && !is_int($id)) {
    ddebug_backtrace();
  }
}

You can also use debug_print_backtrace() if you don't have devel.module but this is much easier to read.