Turns out Plesk, Virtualmin and Webmin all support SNI.
But as to your original question, the latest data I can find shows XP at 39% of the desktop market and IE on XP at 47%, so you still have close to 20% who are going to have problems with SNI.
XP was losing share rapidly, but I think that the release of Windows 8, paradoxically, will keep people on XP longer. Win 8 is still under 2% and I bet if Win 7 were still the standard, you would have seen a greater increase in non-XP versions of Windows.
So for right now, I think you have to pony up and buy IPs. For me, I was just hoping to use it to lock down admin areas, so I would just use self-signed certs and modern browsers, so it wouldn't be an issue, but the hassle is too great for me for now.