Yes you can
http://en.wikipedia.org/wiki/Windows_Small_Business_Server
I noticed something wrong on the SBS 2008 wikipedia entry. You can now indeed use the same service packs. Technically since post SBS 2003 R2 era you no longer need unique service packs for SBS. I guess I can put that but in case folks don't read the R2 part in it, it is properly easier to say SBS 2008. Now SBS uses update rollups to specifically patch unique parts of SBS. All other service packs for the parts of SBS can be applied just like the normal server parts.
Read the complete post at http://msmvps.com/blogs/bradley/archive/2009/06/01/yes-you-can.aspx