well I didn't sign because my taxes were filed electronically.
You sign your electronic tax return by either using a Self-Select PIN for e-file for a completely paperless return, or by signing Form 8453, U.S. Individual Income Tax Transmittal for an IRS e-file Return.See " If the return is electronic, how do I sign it?" for more information.
After you sign the return using a Self-Select PIN or Form 8453,the ERO transmits the return to the IRS or to a third-party transmitter who then forwards the entire electronic record to the IRS for processing. Once received at the IRS, the return is automatically checked by computers for errors and missing information. If it cannot be processed, it is sent back to the originating transmitter (usually the ERO) to clarify any necessary information. After correction, the transmitter retransmits the return to the IRS. Within 48 hours of electronically sending your return to IRS, the IRS sends an acknowledgment to the transmitter stating the return is accepted for processing. This is your proof of filing and assurance that the IRS has your return information. The Authorized IRS e-file Provider then sends Form 8453 to the IRS.
Taken right out of the IRS website.
Authorized IRS e-file Providers for Individuals