Is it permissible under the law to have a Roth-only 401(k) plan? No, according to final regulations issued by Treasury and IRS today. Here is the pertinent excerpt from the preamble to the regulations addressing the issue:
Some commentators requested that an employer sponsoring a qualified cash or deferred arrangement be permitted to offer only designated Roth contributions. However, under section 402A(b)(1), designated Roth contributions are made in lieu of all or a portion of elective contributions that the employee is otherwise eligible to make under the cash or deferred arrangement. If a cash or deferred arrangement offered only designated Roth contributions, an employee participating in the arrangement would not be electing to make such contributions in lieu of elective contributions he or she was otherwise eligible to make under the plan. Thus, these final regulations clarify that, in order to provide for designated Roth contributions, a qualified cash or deferred arrangement must also offer pre-tax elective contributions.
The press release announcing the issuance of the final regulations is here. Roth 401(k) final regulations are here.