Earlier on, I reported the new possibility to trace the start of a checkpoint from within profiler. I tested that new event with manual checkpoint-statements, because that's exactly what happens in the production environment of our application where we force them.
Anyway, we just discovered that with a 'normal' checkpoint, triggered by the database engine itself, the event isn't raised. You can easily simulate this behavior by comparing a profiler trace with the perform counter MSSQL:Buffer Manager-Checkpoint pages/sec on a database with some load. Compare the periodic checkpoint with one you forced yourself. Weird and disappointing.