I won't believe this until I see an official announcement from AMD (not from ATI). An announcement made by someone other than a marketing guy. And that looks like this:
AMD is pleased to announce that the driver code for all its cards will be released under open source license x. We look forward to working directly with the xorg and kernel teams, and hope to get our drivers into the kernel and xorg as soon as possible.
I do however believe that AMD may do this, because when you consider very large rollouts in offices etc, those types of systems will almost never use standalone gfx cards, and with AMD's plans to more deeply integrate graphics into multicore cpus, along with standard cards or mobo gfx chips, the only real choice is to open the specs, and hopefully the drivers, especially if AMD plans on competing with Intel for real long term.