There is nothing built into Dreamweaver that will allow what you're trying to do. Backing up of sites isn't done automatically (as in exporting the sites to .ste files, which are XML files), although theoretically, a Dreamweaver extension could be made to automate the exporting on a specific schedule (assuming Dreamweaver is open), but you'll run into an issue when you try to import the sites on your laptop, unless you first remove the site definition on the laptop prior to the import due to potential naming conflicts, and you're sites will have to be set up in the exact same location with the location existing when you import the sites, or you'll have to browse to their location(s). Also, I think that import is limited to one site at a time, but I could be wrong about that.
Sounds like you need to contact Syncplicity and ask for the ability to sync parts of the Windows registry in addition to files. Or submit a feature request to Adobe: https://www.adobe.com/cfusion/mmform/index.cfm?name=wishform Even then, things may not work out well, as the site ftp passwords are encrypted in the registry and I believe that since CS3 the encryption is dependent at least in part on the system that they were stored in the registry with, but I could easily be mistaken on that.
The ftp passwords when exported to a .ste file are encrypted using a different method (at least through CS4) than the encryption in the registry, due to backward compatibility. The following page lists a decode function (in JavaScript) that will allow you to obtain the ftp password once you've pulled their location within an .ste file:
http://blog.affirmix.com/2008/08/28/encoded-passwords-in-ste-site-definition-files/
Since you mentioned the registry, for future reference, the site definitions for Dreamweaver CS4 are found at:
HKEY_CURRENT_USER\Software\Adobe\Common\10\Sites\
CS5:
HKEY_CURRENT_USER\Software\Adobe\Common\11\Sites\
On Mac the Dreamweaver site preference file is located at:
CS4
/[drive label]/Users/[username]/Library/Application Support/Adobe/Common/10/Sites/Site Prefs
I don't have CS5 on mac available, but I'd assume that it would store the site settings at:
/[drive label]/Users/[username]/Library/Application Support/Adobe/Common/11/Sites/Site Prefs