I'll refrain from speculating about why Apple is waiting until next year for something really great. I do hope it has to do with Haswell though.
The complicating issue is that Thunderbolt not only carries high-speed PCIe data, but must also carry DisplayPort video as well. On all other Macs, GPUs—whether integrated or discrete—are fixed. This makes it easy to pipe the DisplayPort output to the Thunderbolt port, which serves as both a high-speed interconnect as well as the connection for an external monitor. The Mac Pro, on the other hand, has removable PCIe-based graphics cards. How will Apple get the output of these cards into the Thunderbolt controller?The most likely solution is a Mini DisplayPort passthrough cable. ASUS is using an external DisplayPort cable to add Thunderbolt to its latest motherboard designs, but that seems decidedly "un-Apple-like." There may be a more elegant solution in the works, such as directing the card's output over the PCIe bus directly to the Thunderbolt controller, but according to our sources, no current graphics cards work that way. Given that reality, we think Apple will use an internal cable combined with GPUs featuring an internal mini-DP connector.