After experimenting a bit further, I am happy to see that there is 2-way synchronization for Calendars and Contact Lists between SharePoint 2007 and Outlook 2007.
Unfortunately, this does not work with Outlook 2003, as confirmed in another blog:
“Outlook 2003 can link to SharePoint Calendars and Contact Lists on your new MOSS 2007 server and download read-only copies of that content. Outlook integrates with MOSS servers in exactly the same way as any SharePoint server, so there’s no additional limitations over the read-only nature of Outlook 2003’s sync’ing feature. Upgrade to Outlook 2007 to get the full 2-way sync experience against MOSS.
Andrew Ash
Microsoft Outlook“