Using the SaveAs command is not recommended when saving them to earlier versions. Doing so will typically cause the user to see the objects as proxy objects. This is because the earlier release does not understand the AEC object makeup from the newer release.
Because this question comes up every time a new file format is released by Autodesk, we felt it appropriate to put the answer out there before you even ask. With the 2010 AEC product line now on the street, we have confirmed that there are new file formats for AutoCAD Architecture 2010, AutoCAD MEP 2010, Revit Architecture 2010, Revit Structure 2010, & Revit MEP 2010. So what does this mean for someone working with consultants still on earlier releases?
For the AutoCAD based products, when a file is saved to a former file format, 2010 to 2007 in this case, all AEC functionality is broken. I know what you're thinking, "In English please". This means that objects such as walls, windows, doors, ductwork and piping lose their function as objects when their saved down. There are two primary ways you can save a file down to an earlier release; one is recommended, one is not. The first way is to do simply a SaveAs on the drawing and change the format to 2007 in the save dialog box as shown below.
The other way you can save a drawing down to an earlier release is an Export. This basically "flattens" the drawing while maintaining the graphical integrity of the AEC objects. This is the recommended workflow for users needing to send their AEC drawings to a consultant or client working on an earlier release. See the image below for where the Export commands in 2010 can be found.
Now let's discuss the Revit based products. As with all major Revit releases, there is no backwards compatibility. If you've been working with Revit for more than a year this should not come as surprise. As with previous major Revit releases, upgrading to 2010 is recommended to be deferred until existing projects are completed unless all team members are upgrading. Through the course of the year Autodesk releases new builds of the Revit products (otherwise known as Web Updates). Up until very recently these "Web Updates" have been full installs instead of a service pack like the AutoCAD based users are familiar with. That all changed with Web Update #2 for the 2009 Revit products. Web Update 2 & 3 for 2009 were true service packs although they were still not able to appended or merged with network deployments like the AutoCAD based products are able to. It has yet to be seen whether or not that will change with the 2010 web updates, only time will tell.