By default this feature is not enabled in SBS 2008, as Exchange and WSSv3 are installed on the same box. It can nevertheless be enabled by configuring an email domain to be used by the SharePoint site and a foreign connector on Exchange to take care of the mail routing (using Foreign Connector Cmdlets) to a drop folder (used by the SharePoint Site).
Configuring Incoming emails for Sharepoint on SBS 2008: http://www.mindwatering.com/supportref.nsf/c59b2b31baccdc2185256d4300106099/6cfe97df8868dc888525759e00217aa5!OpenDocument
About Exchange Foreign Connectors: http://technet.microsoft.com/en-us/library/aa996779%28EXCHG.80%29.aspx
About Foreign Connector Cmdlets: http://technet.microsoft.com/en-us/library/bb124003%28EXCHG.80%29.aspx
Below I found some related posts:
Here is an interesting post with complete instructions step by step from start to finish to configure an external device to use Exchange on an SBS in order to send emails to Sharepoint libraries: :
Configuring Incoming emails for Sharepoint on SBS 2003: