I'm looking for a way to create a Smart Collection in Lightroom 3 that displays photos that are found in Previous Import and don't have a keyword. The latter part is not a problem, but I can't figure out how to filter for photos that are recently imported. Is there a way to do this in Lightroom 3?
No, Lightroom doesn't let you check for being in the Previous Import in the filters for a smart collection.
If your goal for the smart collection is to show only images that you need to add keywords to then the most reliable workaround I know of is to setup your import process to automatically add a keyword to indicate that you haven't added your keywords to the image and create a smart collection to filter on that automatically added keyword.