ATI's R580+ is to use the same 90 nanometer marchitecture as the current generation, but ATI will be making modifications to it, allowing it to utilize DDR4. Allegedly ATI's, R520, 10k Generation will be moving to DDR4, but a few modifications need to be made to accommodate this change. The launch dates of the next two big cards R600 and G80 is still unknown, but if NVIDIA pulls a fast one on ATI, ATI may be in for some trouble only having a R580+ ready and not a R600 up against the G80. The R600 is expected to launch near the end of 2006.

Source: techPowerUp! - ATI moving to DDR4
Main Source: the Inquirer - ATI's R580+ prepares for DDR 4