The PS3 news ticker this morning said Batman: Arkham Asylum is due for some free DLC on Sept. 17. In its forums, Eidos has confirmed the package will also arrive for PC and the Xbox 360.
An Eidos forum admin said simply, "I'm happy to inform you, it will arrive at the same time," when asked if the DLC would coincide with the PCs Sept. 15 release and be available for the 360 as well. You can read it here. So, it's a three platform simultaneous release. No other word what the DLC drop contains yet.