The contention that DRM doesn't stop piracy and therefore should be abandoned is false an misleading, and yet it repeated constantly by anti-DRM zealots.
It is absolutely true that DRM will likely never totally prevent piracy. With enough time and effort, virtually any DRM can be cracked. Of course, that's not the reason DRM exists, and that's why the entire line of argument is a straw man.
DRM very effectively prevents casual piracy and counterfeiting. Windows Activation, for instance, effectively prevents the average computer user from sharing their copy of Windows with their buddy, as well as allows users to accurately tell if they were sold a counterfeit copy.
Does the application of DRM to a software product result in the overall piracy rates of the product changing? That's a hard question to answer if only because casual piracy is virtually impossible to track.
It's fairly easy to track torrent downloads, but no so easy to track how often somebody casually lends an installation CD to somebody else. Studies of traditional piracy suggests that applying DRM doesn't affect the types of piracy that are easy to keep track of publicly.
Casual copying can really only be detected if the product in some way communicates back with the company that created it, or if there are mechanisms in place to audit large numbers of users in some way. Indeed, companies like Microsoft do both of these things. Microsoft claims that Windows Activation in Vista reduces casual piracy and counterfeiting by 50% over XP's Activation process. Even if that figure is off by an order of magnitude, it still makes a lot of sense for Microsoft to pursue these measures.
In the case of digital content distribution software, like Apple's iTunes or Microsoft's Zune Marketplace, DRM may do very little to decrease piracy overall, but these services simply wouldn't exist without DRM. Content creators (record labels, TV producers, etc) would not allow Apple or Microsoft to distribute their content digitally without some safeguards against trivial and casual piracy of that content. It's one thing for somebody to buy a CD, rip it, and create a torrent of it, but it's a far bigger problem if that same person had unrestricted access to 2 million songs that they could then download and redistribute and at ease. Whether that same content is available in bits and pieces via piracy sites is not the question (it obviously is), it's the ease of access and redistribution that a something like iTunes or the Zune Marketplace would provide to would be pirates if there was no DRM applied to those services.
DRM is meant to raise the barrier just enough to prevent casual piracy ("don't copy that floppy!"), and to make media companies happy enough to allow digital music companies like Apple and Microsoft to provide huge content libraries for distribution. It seems that, for now, it is accomplishing these goals. If it weren't, that funky thing called economics would dictate that companies stop using it.
I realize that anti-DRM zealots really want to make as big a stink as possible, either because of some silly ideological stance or because of a previous bad experience. Fine, if you manage to convince enough people that DRM is evil, maybe companies will stop using it. But stop saying that it doesn't work. It does. And, honestly, most computer users couldn't care less about DRM as long as it doesn't get in the way of them doing things they think they shouldn't reasonably be allowed to do.