I've checked the problem comes since http://code.google.com/p/web2py/source/detail?r=9153fbe0349812922daae09c487f4dfe68280d76&path=/gluon/tools.py where the user_id = table_user.insert(**keys) replaced the old method to insert data in the auth table.
No idea of the rationale behind the change, but it's causing this problem.
