Also having 280 CPUs in a cabinet yet only having 28 power cables (2 per enclosure, assuming you went with the redundant power supplies) is really nice. I've dealt with a full-up (or nearly so) cabinet of 1U machines and the power cord issue (not to mention supplying that much amperage) is a nightmare.

Plus, the built-in NICs, apparently attaching to the enclosure's backplane, is a good way to avoid an ethernet patch nightmare.

I'm not really sure what you mean by hot-plug, though? You want the enclosure to run as an SMP or NUMA machine and remove CPUs while running? You're going to pay a lot more for a multi-processor solution, and well more than that for being able to remove CPUs on the fly (not even sure if there's an Intel solution where you can do that at all).

Speaking of which, this always reminds me of when Microsoft claimed that the eBay failures were Sun's fault. My favorite (partial) rebuttal:

Microsoft says, "System boards that are hosting non-pageable kernel data structures cannot be removed from a domain without interrupting service. The Solaris operating system has to undertake a special "quiesce," or suspend, operation while the critical pages are migrated to another board." The very fact that you can migrate kernel memory off of a certain piece of hardware and swap it out is pretty incredible. It is also a feat that Windows NT cannot do under any circumstances. As a slashdot reader noted, "This is a bizarre attempt at deception akin to buy my car -- that brand sucks because you can't change the brake pads while it is speeding down the Interstate!"

(From http://www.solariscentral.com/news/commentary/ebay.shtml)