Here is how FairPlay works: When you buy songs at the iTunes Music Store, you can play them on one — and only one — line of portable player, the iPod. And when you buy an iPod, you can play copy-protected songs bought from one — and only one — online music store, the iTunes Music Store.

The only legal way around this built-in limitation is to strip out the copy protection by burning a CD with the tracks, then uploading the music back to the computer. If you’re willing to go to that trouble, you can play the music where and how you choose — the equivalent to rights that would have been granted automatically at the cash register if you had bought the same music on a CD in the first place.

Even if you are ready to pledge a lifetime commitment to the iPod as your only brand of portable music player or to the iPhone as your only cellphone once it is released, you may find that FairPlay copy protection will, sooner or later, cause you grief. You are always going to have to buy Apple stuff. Forever and ever. Because your iTunes will not play on anyone else’s hardware.

Unlike Apple, Microsoft has been willing to license its copy-protection software to third-party hardware vendors. But copy protection is copy protection: a headache only for the law-abiding.

Microsoft used to promote its PlaysForSure copy-protection standard, but there must have been some difficulty with the “for sure” because the company has dropped it in favor of an entirely new copy-protection standard for its new Zune player, which, incidentally, is incompatible with the old one.