I'm on Moodle 2.2, looking at the Web-service documentation at 'Site administration > Plugins > Web services > API Documentation'. It says that the argument to 'core_user_create_users' is an object with fields such as 'username', 'password', and 'firstname'. So is the result returned by 'core_user_get_users_by_id'. But there are odd differences.
For example, the parameter can contain an email address, but not a Skype ID, Yahoo ID, or MSN number. The result can contain all of these. So you can get these fields, but not set them.
Also, the parameter has this field:
description string // Optional // User profile description, no HTML
But the result has these fields:
description string // Optional // User profile description descriptionformat int // Optional // User profile description format
So it looks as though there's a flag to describe a profile's format — but you can't set that format!