According to what information I have been able to glean from Microsoft about the impact on XP on Win2K certifications, I can state the following:
1. The Win2K certifications should remain valid at least until the end of 2003. 2. It will be possible to mix and match Win2K and XP elective exams when obtaining an MCSE; however, all core exams and elective core exams must belong to either the Win2K or XP tracks.
Thus, I don't believe XP will influence your certification much for at least another year and a half (perhaps you'll have even longer before Microsoft requires another MCSE upgrade).
This was first published in June 2001