Can SharePoint Designer 2010 be used against Sharepoint 2007 Sites?
No, Microsoft SharePoint Designer 2010 cannot open Microsoft SharePoint 2007 sites.
I guess this is not really suprising. The good news is that SharePoint Designer 2007 and 2010 can be installed on the same PC – just remember to install 2007 first.
This blog post relates to Microsoft SharePoint 2010 beta.