That infamous Xbox 720 document — which Microsoft has been busily taking down from any website that has posted it — is impossible to link to directly when shared on Dropbox. First spotted by WPCentral, this behavior seems to have been prompted by Microsoft laying a copyright infringement claim over the document, and is the first time we've seen public links to a file (of any kind) being restricted by the cloud storage service. To be clear, it doesn't appear that Dropbox is actually reading or inspecting the content of the file, as we were able to upload a version of the Xbox 720 doc with a couple of slides removed and successfully share a link to that altered PPTX file. The most likely scenario is that checksums are being compared, and when Dropbox finds one that matches its blacklist, it refuses to share a public link. Sharing via the usual Dropbox method of having shared folders between accounts still works as usual.
Dropbox's behavior in these circumstances is perfectly in line with the company's terms of service, which state that "Dropbox will respond expeditiously to claims of copyright infringement committed using the Dropbox service and/or the Dropbox website," and also forbid users from publishing anything that "infringes another's rights."
Rather ironically, Microsoft's own SkyDrive cloud service doesn't prevent you from sharing a public link and preview of its revealing PowerPoint presentation.