We've clocked the latest 2.5" SSDs at nearly 550MB/s in sequential transfer rate tests. That speed isn't far off the peak theoretical transfer rate of the 6Gbps Serial ATA interface used by modern drives. Next-gen SSDs will need a new interface to really stretch their legs, and one is on the way from the SATA-IO group responsible for the Serial ATA specification. Dubbed SATA Express, this new standard has reached the ratification stage (PDF) and is currently being reviewed by SATA-IO members.
As the name implies, SATA Express combines Serial ATA and PCI Express. The interface is wholly PCIe-based, but the physical connector borrows elements from the SATA world. The idea is that systems will be able to offer a single physical port that's compatible with both SATA Express and traditional Serial ATA devices.
With no translation overhead, SATA Express should allow drives to access the full bandwidth available in PCI Express. The initial implementation will work with PCIe 3.0, which offers 1GB/s of bidirectional bandwidth. Drives that connect via cables are expected to offer one or two lanes of bandwidth, while card-based implementations will boast up to four lanes. Either way, you're looking at a substantial leap over the 600MB/s ceiling of 6Gbps SATA.
Unfortunately, there's no word on when we'll see the first SATA Express-compatible devices. The specification is expected to be completed some time this year, and I'd expect products to follow shortly thereafter. Thanks to X-bit Labs for the tip.