EE Times goes into a little more detail with its coverage, pointing out the differences between the new spec and DDR-II:
There are two distinct differences that set GDDR-3 apart from DDR-2. One is the use of a single-ended, unidirectional strobe that separates the reads and writes. DDR-2, by contrast, uses differential bi-directional strobes.GDDR-3 is an open, royalty-free standard, and NVIDIA actually participated in its development. Since GDDR-3 is really only designed for graphics implementations, JEDEC ratification shouldn't matter, just as long as graphics manufacturers control which memory chips are paired up with their GPUs.
The second change made in GDDR-3 is the use of a "pseudo-open drain" interface technique that is based on voltage rather than current. This was done so that graphics chips can be compatible with DDR, DDR-2 and GDDR-3. Like DDR-2, GDDR-3 interface uses 1.8-Volt SSTL.