Google Maps has finally added two new features collaborative editing and importing of KML/ KMZ files. If you create a map in the "My Maps" section, you can now invite people to collaborate or allow anyone to edit the map like in a wiki.
Google Lat Long blog explains:
The My Maps feature of Google Maps lets people create maps to share their hobbies and expertise with the world….
….multiple people can edit the same My Map. Just click the "Collaborate" link and enter the email addresses of the people you want to invite. They'll receive an email invitation with a link to the map. Once they open the map, they should be able to edit it, as long as they are signed into a Google Account that's associated with that email address. You can also open your map to the world so anyone can edit it by selecting the "Allow anyone to edit this map" checkbox.
Brian Cornell at Google Lat Long blog, details how to import KML (Keyhole Markup Language) files:
Maps, Google Maps, New Features, Featured News, Google
If you use Google Earth, you're probably familiar with the KML and KMZ file formats, which are used to store geographic information like placemarks, lines and shapes. Now you can share your KML files more easily with other people (who may not have Google Earth installed) by importing them into My Maps….
To import a KML file, create a new map and click the Import link. You'll be able to upload a file from your desktop, or enter the URL of a file on the web. We'll import all of the data from that file into your map for online access and editing. Keep in mind that Google Maps doesn't support as many features of KML as Google Earth, so some of your information (like 3D models) won't be imported into your online map.