Hi Ish.
I've checked this out. It appears the slide Technet details for this have changed. Having installed the product and tested, there is no "built in Antispam" software other than the native stuff found in Exchange 2010. That may well suffice for a small company of few users, but I would be inclined to look at a 3rd party app.
Unfortunately, I',m not sure ForeFront for Exchange would run competently on SBS 2011. It would need to be a serious spec'd machine. Any of the other 3rd party style apps suitable for 2008 R2 should suffice. Trend Micro Worry Free, etc.