In FCP7, do a Command-A to select all of your bins and sequences. Go to File... Export... XML. Choose "Apple XML Interchange Format, version 4". Deselect "Include Master Clips Outside Selection" (because there shouldn't be anything outside your selection anyway). Select "Save project with latest clip metadata".
Open the resulting XML file in FCP 6 by first creating a new project, then choosing File... Import... XML. Leave all settings intact, and wait a few second for it to import. The bins should all be preserved nicely, but the files are offline. If you get a warning dialogue about errors, that should be fine.
Hit Command-A to select all of your bins and sequences, and then choose "Reconnect Media" from the File menu. Once you've reconnected everything your project should be pretty similar to the FCP 7 project.