All the threads on the "openness" of drivers quickly turn into a religious war, and it's hard to find real information. Having just watched the video from Maker Faire, Eben provided a nice answer in the Q&A section and the real situation is a lot better than I think many realise.
Here's the transcript, with slight editing for readability.
Feel free to copy/paste it to the wiki/FAQ if you want.
OK, so the question was: "Are all of the drivers open?"
We classify bits of the chip into "multimedia side" and "ARM side". Everything on the ARM side is definitely open. So SD Card drivers, USB drivers, etc, all of that is open. On the multimedia side it's a little bit more complicated because we have a proprietary GPU design inside there that has a closed binary blob associated with it - not in ARM code but a closed blob of GPU firmware that runs on a proprietary RISC core. There are no compilers available [publicly] for that RISC core, so we're unlikely to ever release the source to that.
There's a sort-of middle ground which is what happens to the [linux components]. We have a kernel-side component that provides a tunnel between user-side OpenGL drivers and the closed GPU design. Obviously the kernel-side component is open as it's statically linked with the kernel so it has to be for GPL reasons. The userland component? I'm doing my very best to make that open. I think it would be interesting to make it open because I think it would be interesting for people to see what a commercial OpenGL state machine looks like. I'm doing my very best to make sure that's open. It's possible that in the initial release those will be closed ARM shared objects but we acknowledge that that's not ideal.