Those of you who are making this argument, please think about whether you support the idea of corporate personhood and whether you believe that there is a corporate right to free speech and to decide which customers they want to service and which ones they don't.
Unfortunately, this isn't a black and white issue. The founders were all about free speech in the federal government, but the original intent of the constitution was to allow the states to make their own laws that might place additional restrictions on peoples' rights. This turned out to be a very bad idea (slavery, segregation, etc), and the constitution was eventually amended to allow the Federal government to step in and enforce the bill of rights in a way that would *supercede* state laws. But to say the the founders would have necessarily supported forcing a private company, Rackspace, to broadcast someone else's speech is just wrong. They weren't even in favor of forcing the *states* to allow free speech.
All that being said, it's a very legitimate idea that the right to express oneself supersedes property rights, *particularly in cases where there are few options for expressing yourself*. This is the whole idea behind Net Neutrality. Since the Internet is controlled by a few large companies, at some point it becomes necessary to step in with legislation and preserve the *de facto* right to free speech, since if these few private companies decided to arbitrarily restrict political speech (or any other kind of expression) it would put a serious dent in our ability to express ourselves.
All told, Rackspace's obligations to respect the free speech of other groups ought to be tied directly to how many other real, useful alternatives exist to their service. If it would be relatively easy for this site to pick up and move to another server farm, then I think Rackspace's policy ought to stand. On the other hand, if all hosting providers have this policy, or Rackspace is the only one big enough to handle hosting their website, then it becomes a Net Neutrality issue.