A small fraction of Internet users on extremely out-of-date system cannot use SNI. If you need to support those users, you will still have to buy a dedicated IP address. I personally don't bother (if you're running IE 6 on Windows XP, you have bigger problems).
This is a great guide! I did the exact same thing several months ago and am very happy with the result. Enough with the excuses not to deploy SSL!
[1] https://blog.webfaction.com/2011/12/server-name-indication-e...