For those of you who have tried installing Exchange SP2 on SBS 2008 but have had it fail during its initial steps, this blog post is for you!
Microsoft has created a tool that you can download and install which permits you to install Exchange SP2 on SBS 2008.
For more information on the procedure and to download the tool please see:
http://support.microsoft.com/kb/974271/
I cannot stress enough on the importance of a backup in case things go wrong. I have performed this at numerous client locations, most successful; however in one instance while SP2 was installing, the update failed and totally removed Exchange from SBS 2008. This was unrecoverable and a full restore from a backup would have been needed (thankfully this was the configuration of a new server so we just restarted the implementation).