Besides open-sourcing their driver stack under a three-clause BSD license, they are also providing full documentation on their VideoCore IV graphics core. The driver code targets the BCM21553 SoC but Eben says it should be straightforward to port it to Raspberry Pi's BCM2835. The Raspberry Pi Foundation has offered $10,000 USD for the first developer to port the code to the Pi so it can run Quake III on a fully open-source stack.
More details on today's open-source Broadcom Core IV milestone can be found via the post at RaspberryPi.org and the Broadcom blog. Two years ago they open-sourced some graphics code that turned out to be disappointing but at least it finally looks good now with this round of driver code and documentation.