Another practice mentioned several times at the recent connect conference was to designate a "master" of the NameMapping for your team. That person would be responsible for maintaining and developing the object mapping. This could take various forms, two of which are:
1) The person does ALL the mapping. For every test case, the person would map all new components that are needed, check the altered NameMapping file into the source control, then everyone would retrieve that file and utilize the mapped objects.
2) Everyone maps their own components. Then the master uses the built in "merge" option to merge all the separate files into a single file, uploads that file to source control which then everyone retrieves for their work.
tristaanogre's #2 is what we use with 4 people and it works pretty well. I wouldn't go more than a couple of weeks between merges or it gets too messy. Our teams with more than 4 people use approach #1.
