Ahh, good. I'll register Rackspace as my tech conference, because that's a totally separate industry than Rackspace the infrastructure provider.

It is a totally separate industry.

One Rackspace would be in the industry of technology; the other in the industry of hosting conferences. The likelihood for confusion arising from the conference having technology-related subject matter is merely a factor in determining whether the conference would violate Rackspace's trademark. It's very likely that a trademark violation would be found, but it's still possible that a violation wouldn't be found.

Note also that Rackspace only trademarked "Rackspace," so "RackSpace," "Rack Space," and "Rack-Space" are separate marks a tech conference could use those without infringing upon Rackspace's trademark.

Yes, but tech companies (among others) also run conferences specifically for their users. I suspect Rackspace would take action if you wanted to run the "Rackspace Summit" or something along those lines because people would reasonably assume that such an event was associated with the company in some way.

(Companies do often allow quasi-independent organizations to run quasi-associated conferences but this is usually a deliberate decision.)

I was primarily responding to the comment that they were not separate industries, and supplying an example of similar marks that were not actually trademarked by Rackspace and thus open to potential use even by those in the technology industry.

Legally, technology and conferences are entirely separate industries with very different business, legal, and tax considerations. A technology company that makes software for conferences is not in the conference industry; a conference with technology subject matter is not in any of the technology industries. This may be splitting hairs, but the law is always a matter of splitting hairs.