According to Yanes, the delay is necessary to ensure that PCI-E 3.0 retains backward compatibility with previous standards. Given the delay, products using PCI-E 3.0 won't make it to market until 2011. This shouldn't be a problem though, being that most mainstream GPUs can't saturate the bandwidth PCI-E 1.0 (or 1.1) has to offer, much less PCI-E 2.0.
PCI-E 3.0 boosts each lane's transfer rate to 8GT/s and has a 1GB/s data rate - it effectively doubles the bandwidth of PCI-E 2.0. The new spec also up the data-encoding scheme from 8/10-bit to 128/130-bit, and data transfers have little to no overhead - compared with the 20% seen on previous versions.