Not sure if this is a little more clear in later releases of Symfony, but in 1.0.x the load-data task, as outlined in the documentation and cookbook, is somewhat vague.
If you’re appending a fixture to an existing database, here’s what you want to do:
$ php symfony propel-load-data <APPLICATION_NAME> [<ENVIRONMENT_NAME>] <FIXTURES_DIR_OR_FILE> append
so usually I do something like this:
$ php symfony propel-load-data frontend data/fixtures/import_data.yml append
I’m actually not sure how ENVIRONMENT_NAME would come in to play.. Either this is a way to select per-environment fixture data (probably), or per-environment databases to which to apply the data.