The SharePoint Infrastructure Update was released yesterday. My advice: install SP1 and then install the Infrastructure Update IMMEDIATELY! There are a lot of bug fixes and features in both of these updates that can make your SharePoint experience much better.
The updates can be downloaded from the links below:
Infrastructure Update for Microsoft Office Servers (KB951297) - x86
Infrastructure Update for Microsoft Office Servers (KB951297) - x64
Infrastructure Update for Windows SharePoint Services 3.0 (KB951695) - x86
Infrastructure Update for Windows SharePoint Services 3.0 (KB951695) - x64
Infrastructure Update for Microsoft Office Project 2007 (KB951547) - x86
You can read more about the release announcement on the SharePoint team blog here.