My gut says they're slow-playing what's expected to be another record-setting haul in case they need to trump some good news for McCain. They didn't need it for McCain's retooling, or even his re-retooling, the reaction to which ranged from yawns to outright disdain. But if McCain turns in a good performance tonight, I'd expect to hear the September total some time tomorrow.
Update: The commenters make a great point: One reason not to announce your September haul is that you don't want your donors getting complacent. On the other hand, the numbers are due at the FEC on October 20, so Chicago can't keep them quiet forever. I'd guess they sit on them till the deadline unless they need a conversation-changer beforehand.
--Noam Scheiber