The Inq revealed ATI's upcoming 80nm R580+ GPU will feature support for GDDR4 memory.
All cards after the R520 could support GDDR 4, but obviously the next generation GDDR 4 memory will simply work better with the new chips. The new Radeon X1000 generation memory controller has the support but obviously R580+ will have even better and more polished support.
Companies such as Samsung and Hynix have been sampling GDDR 4 from late 2005, so I guess by the time the 580+ is ready in late summer, there are unlikely to be many problems with memory shortage. GDDR 4 is a progression from GDDR 3 memory with some tweaks, squeaks and extra whistles.
NVIDIA may start using DDR4 memory with their G80 GPUs.