You can’t edit any of the feature classes in the updated feature dataset until the feature dataset is re-registered as version again.
If you add a new feature class to the feature dataset after you have already registered the feature dataset as versioned, the new feature class are not registered as versioned automatically.
Individual feature classes cannot be registered as versioned within the feature dataset. Only feature dataset are registered as versioned so you will need to re-register the feature dataset again after adding the new feature classes to it.
The existing feature classes will remain registered as versioned. Registering the feature dataset as versioned again simply registers any new feature classes as versioned.
ArcGIS 10 References:
> Register As Versioned geoprocessing tool [ArcEditor/ArcInfo licences only]
> Registering data as versioned Help