Centro is a sort of the new version of SBS, but possibly a bit more flexible, targeted to businesses with less than 250 computers. I always felt SBS was best for up to 25 ccomputers, the hardware and software too heavy for more, but now you would be able to build a mammoth 64-bit server with giga+ network card that would be able to support more coomputers. What about redundancy? There are answers to this in the hardware department (not cheap though) and Microsoft will no doubt have answers.
More about this and beta enrollment here: