you have until the end of this month to purchase the Premium Add-On. It gives you a second server license as well as SQL 2008 R2. You'll need to buy Premium CALs for each user that will be accessing the SQL application. the other users won't need the CALs.
While you CAN install SQL full on SBS 2011, Microsoft recommends you don't do it unless your software vendor is willing to support the SQL and application on the SBS 2011 server. If not, you get a server standard 2008 R2 license with the Premium Add-on, so you can spin up a new server if you need to.
Kevin
