There's still quite some time before we will see the ARM Mali-T604 graphics processor in any device paired with an ARM Cortex-A15 CPU, but the information we have received indicates that the driver situation won't be radically different from current Mali offerings. Like is the case right now, there should be the open-source X.Org (DDX) driver with EXA/DRI2 support. However, for a working 3D driver stack, you will need the updated Mali DDK that is provided to just the Mali GPU customers along with their closed-source MaliDRM.
It would be nice if the situation would improve where there was an open-source 3D driver (ideally based upon the Gallium3D architecture) with an open-source DRM (Direct Rendering Manager) module with aims towards mainline Linux kernel integration, but it doesn't look like we will see that for the Mali-T604 or at least not immediately. This is pretty much the same situation across the embedded graphics world where the Linux graphics drivers are largely dependent upon binary blobs for all but basic 2D/X.Org support.
In July there was an open-source 2D/3D kernel driver by Qualcomm for their Snapdragon, but that unfortunately didn't move anywhere but it did cause a dirty mess due to the lack of an open-source user-space driver (more details on the situation). Intel also has their GMA 500 Poulsbo / GMA 600 Moorestown graphics IP licensed from Imagination Technology based upon the PowerVR SGX core, and we all know what a mess that is.
The ARM Mali-T604 looks like an especially nice graphics processor for the embedded world, but unless things magically change before the hardware starts shipping, there isn't expected to be a fully open-source Linux driver stack. Those interested in reading more about the technical features of the Mali-T604, hit up the ARM product page.