Field negation is outlined in the mongo docs here: http://www.mongodb.org/display/DOCS/Retrieving+a+Subset+of+Fields

It is supported in the core of mongo but not in the Java Driver.

For example:
BasicDBObject fields = new BasicDBObject();
fields.put("somebigfield", "0");

DBCursor cur = collection.find(new BasicDBObject, fields);

Will return the "somebigfield" rather than omitting it. It doesn't matter what value is given for the field value (0, 1, xxxx) the same thing is returned - ie. somebigfield and the id