Windows XP Professional x64 Edition RC2
It looks like that during my studying period Microsoft released a second release candidate (RC2, build 1433) of 64-bit Windows XP Professional x64 Edition. If you are lucky enough to be a beta tester (and have compatible 64-bit CPU) you can download it from here. The rest of us will have to live with RC1 (build 1289) which is available as a free download from Customer Preview Program web page. Luckily the release date for final version is not far away. We can expect it somewhere in April.
UPDATE: It looks like Microsoft has updated the free download of Windows XP x64 Edition on CPP webpage to version RC2, build 1433. Or at least the file name suggests that: w2k3sp1_1433_usa_x64fre_pro.iso