ATi has pledged to open the specs and to pay programmers to provide a basic drivers, and the remaining work is left to the community. It seems like this is the way things are working. We have most things working as well as the current Mesa limitations allow (e.g. OpenGL 3+ are Mesa-bound) with 4xxx hardware, and the community is taking over a lot of the Gallium3d work and improving the driver.
I can only speculate, but it seems very likely that AMD will provide enough specs and code to ensure that we will have a fully-featured open source 5xxx driver, minus the DRM-infected bits such as hardware video decoding (which will be done on shaders). It's likely that initial support will be coming soon and take a while (year?) to mature.
It's very likely that the performance will always trail behind the proprietary blob because it lacks 15 years of accumulated driver optimisations, but it should be usable for most work.
Hope that helps.