The PCI SIG confirmed the PCI Express 3.0 standard has been delayed from this year to 2010 due to issues with backwards compatibility. Previously, the first PCI Express 3.0 devices were expected in 2010, but we'll now have to wait until 2011 to get our hands on them.
PCIe 3.0 ups the bus' clock to 8GHz. It also ups the previous version's data-encoding schemes fro 8/10-bit to 128/130-bit. One upshot of changing the encoding scheme is that the cost to data transfer is reduced from 20 per cent to around 1-2 per cent, ensuring more real data is sent at any given clock cycle.
That means PCIe 3.0 will have an effective bandwidth of 7.99Gb/s, compared to the 5Gb/s-rate PCIe 2.0's effective bandwidth of 4Gb/s. That's a doubling of bandwidth without the need to aim for 10Gb/s with all the greater signalling issues that 10GHz brings over 8GHz.