A Knowledgebase article detailing the contents of O2K7 SP1 had also been made available this morning. Microsoft removed that article, perhaps for editing purposes. The link to the download itself however remained active. The Office service pack is not a beta; it is a complete release.
Early beta testing of XP SP3 revealed new security hardening features were put in place, and at least one group of researchers claims Windows Vista SP1 lags behind XP SP3 in tests involving Microsoft Office workloads. Those tests may not have been deployed using O2K7 SP1. Today's XP service pack download is a release candidate -- past the beta stage, but not yet ready to be declared a formal release for production systems.
Windows XP Service Pack 3 (SP3) includes all previously released updates for the operating system. This update also includes a small number of new functionalities, which do not significantly change customers experience with the operating system.
Update: Download [Official Link]Windows XP, Service Pack, XP SP3, Release Candidate, Security Update, Microsoft, Download