Actually you can download XP SP3 today with some leaks online. Searching on a Bittorrent site, will show that Windows XP SP3 5503 is available for download and this is the build which seems to be the RTM for Service Pack 3 for XP.
After Vista SP1's release, all the people who don't want to leave XP were eagerly waiting for XP SP3. There were a few RC (Release Candidate) released before this one and hence everyone was anticipating a final release (RTM) sometime soon. Microsoft hasn't confirmed that the SP3 build 5503 is the RTM one and maintains the fact that it will be released in the first half of 2008.
And yes, it is the first half of 2008. Just like Vista SP1, this version could well be the RTM. During Vista SP1, Microsoft had released a build about 2 weeks old and given it to a lot of people. After testing when this build (specifically 1840 for Vista), it seemed to be stable enough and Microsoft called this the RTM build. Same could be the thing for XP SP3 and 5503 can be the RTM build, but don't take my word for it... Try it yourself and check back when the actual RTM is out!!