Why introduce a new driver and not integrate into current one? Would ease up hw detection - just everything from ATI vendor pciid to ati driver. Currently you need a list with all supported old cards for ati and you use the rest for vesa. When you want to write a list for new driver then good luck... It is enough to update my own pci id list from fglrx driver.
If you read the press release for the Kernel Summit, they pretty much stated that they want to support Open Source drivers for across the board ATI products, so most likely, yes, that includes R300/400 products too
And what about R100/R200 based products? I assure you that these cards are still out there, and are in active use...
MAYBE they'll merge the current code for R100/200 and the new once they figure out if the new code can drive these cards or if they need some other means to unify them under the same code base, but I'm not sure it'll be a high priority in the devs TODOs, most likely not.