The Create is designed to be an expandable platform, but the core robotic behavior remains intact. Where the NXT builds upon Lego blocks, Create designers have screw mounts, cargo bays and fixed resources to build on. The Create is really a multiprocessor system.
The Create's main processor is the same as the Roomba. It can run simple scripts and its behaviors can be controlled by another processor. Out of the box, the Create has only the main processor, but any work will likely include adding at least one more processor, like the 8-bit Atmel AVR in iRobot's Command Module.
The Create inherits the Roomba's navigation capabilities, and alleviates the need for the programmer to perform those low level chores. The Create has the potential to be a platform for more sophisticated processors, making it a better target for developers looking to create more advanced robots.
White Box Robotics' PC-BOT is for heavy duty research, literally. Compared to the other two platforms, the PC-BOT weighs a ton. Well, not quite...but it is not something you want to pickup repeatedly.
Like the NXT, the PC-BOT has a single processor for handling computational chores. Of course, the PC-BOT has a much more powerful processor. In fact, it can house any Mini-ITX motherboard.
Cost is a major factor when choosing between these platforms. The NXT and Create are a couple hundred dollars while the PC-BOT is an order of magnitude higher; though, of course, it is more powerful by an order of magnitude as well. Its drive train and battery are significantly heftier. The amount of processing power and the number of peripherals that can be brought to bear for a particular problem are also significant.
The other key difference between these platforms and older ones is the quality of the software. But that's another part of the story.
For more information, visit Amtel, iRobot, Lego, Microsoft, NASA, National Instruments, Parallax, Spark Fun Electronics, and White Box Robotics.