Karl Hill gives us a quick reminder of a command that's been around awhile, but appears to be missing from the ribbons in AutoCAD 2010.
"The Revert command, which I use quite often, can still be launched by typing Revert at the command prompt. (It used to be in Express Tools menu>File Tools>Revert to Original.) This command closes your drawing and reopens it at the point of the last Save. You might wonder, Why not just use the Undo command? Unfortunately, that doesn't always produce the expected results, particularly in Civil 3D. Sometimes the Undo command doesn't completely reverse everything you expect it to, since some of the Civil 3D commands have many small subcommands inside them.
"Revert is a great command to use when you want to try a possible design alternative. Always save your drawing at a point that you want to get back to before you proceed. Then you can type Revert to get back, or if you like your newly created design, just save it as usual."
Notes from Cadalyst Tip Patrol: I'm not sure why the Revert command is not in the Express Tools menu in AutoCAD 2010, but it doesn't appear to be there. If you turn the Menu Bar back on, it is found under the Express menu, under the File submenu. This command closes your file without saving it, assuming you have made changes since your last save. When you use the Revert command, you will be prompted to save or close the drawing. Be careful using this command; depending on when you saved last, you could lose more of your work than you remember!