Microsoft will ship the CD to you free of charge. This CD includes the same Service Pack 2 software that is available for download from Windows Update. You'll wait 4-5 weeks for delivery, according to the site. You can also download the complete service pack here.
Note that Microsoft started the electronic delivery of SP2 to Windows XP Home Edition users last week, and to XP Professional Edition today via the Automatic-Updates distribution route.
I received my Windows XP SP CD today.