If you e-file your client's return, your signature is your PIN, not the "signature" on the return. When a return is filed on paper, your signature requirement, as a preparer, is governed by Notice 2004-54.
Under the Notice, preparers can sign "by means of a rubber stamp, mechanical device, or computer software program." It further stipulates that "these alternative methods of signing must include either a facsimile of the individual preparer’s signature or the individual preparer’s printed name."
In other words, your printed name in the signature block is legally sufficient under the Code and Treas. Reg.
Still an AllStar