Microsoft says this about SP Designer 2010:

Early on in development we made a tough choice. Not supporting backwards compatibility with SharePoint v3 and earlier. This means that SPD 2010 will only work with SharePoint 2010. Although this was hard, it allowed us to build better, farther and test deeper. I almost think of SPD 2010 as a new product, a new toolset for SharePoint 2010. This of course means that we are supporting side-by-side installation of SPD 2007 and 2010. There is one important thing to keep in mind when setting up side-by-side. Stick to the same flavor of clients (x64 and x86). SPD 2007 only came in x86, so only use x86 SPD 2010 when installing side-by-side. This is for all Microsoft Office apps as well. So don’t install 64-bit SPD with 32-bit Office.