Wikipedia has a list of software that supports SNI -- Not quite statistics, but a good starting point.
As of October 2010, Tom's Hardware said 66% (2/3) of Windows users were still running XP. I imagine that number has reduced, but I know it's still holding on in medical devices and other "everything-must-be-triple-vetted" type environments.
You also can't neglect corporate proxies that intercept and handle SSL requests -- some of these may not be SNI-capable yet.
To help your decision-making process along, right now the short answer is "If you need SSL to work for everyone you still need one IP per certificate" -- Unless your users are technically savvy enough that they will all be on platforms that can handle SNI (and you can enforce that as a requirement) you don't want to be dealing with the backlash from certificate-name mismatch messages when someone drags out their old XP laptop...
W3schools has some helpful stats on their browser users. The headline for the purpose of this question is that as of April 2011, 40.9% of their users were still using Windows XP. For W3 schools, a lot (42.9%) use firefox, which would support SNI, but I suspect on the wider web, this percentage is likely to be smaller. 24.5% of their users use IE. I would estimate therefore that a useful lower bound for the drop-out rate on w3schools if they were to use SNI would be 10.5% (24.5% of 42.9%). On the wider web, I'd expect this to be significantly higher.
UPDATED AGAIN: 2013 Year End
As of late December 2013, the StatCounter GlobalStats table at Can I use stated that Internet Explorer 8 and older total 8.26 percent, and Android Browser on Android 2.x totaled 1.52 percent. (IE on Windows Vista or later supports SNI, but users of Windows Vista or later have had an opportunity to leave IE 8 behind. Thus I use IE 8 as a proxy for IE on XP.) Because security updates for IE on XP will end in April 2014, we can assume that few people who care about security will be using IE on XP next year.