I blogged about this in 2011, citing to an Office of Legal Counsel opinion that concluded:
[We find that, pursuant to] the legal understanding of the word “sign” at the time the Constitution was drafted and ratified and during the early years of the Republic ..., a person may sign a document by directing that his signature be affixed to it by another. We then review opinions of the Attorney General and the Department of Justice and find the same understanding reflected in opinions addressing statutory signing requirements in a variety of contexts. Reading the constitutional text in light of this established legal understanding, we conclude that the President need not personally perform the physical act of affixing his signature to a bill to sign it within the meaning of Article I, Section 7.
The state of the law surrounding proxy signatures has remained amazingly constant through both English and American history. The proxy and the principal must be present together when a proxy signature is utilized for a high-value transaction. This was the rock-solid law when the constitution was written.
No one seems focused on the presence requirement: not the President, nor the Republican House members who complained to the President, nor the numerous legal commentators and scholars that we have heard from since the auto-pen signing occurred. The use of the auto-pen, itself unproblematic, seems to have eclipsed the more important issue of whether the president was present when it was used.... In sum, the President’s use of the auto-pen (or even a human being) to sign a bill outside of the President’s presence is unconstitutional.
Note that, even if the auto-pen signature were to be found invalid, this would not mean the bill is vetoed (unless it comes to the President near the end of a legislative session); but it would mean the bill goes into effect ten days (not counting Sundays) later, according to article I, § 7 of the Constitution:
If any Bill shall not be returned by the President within ten Days (Sundays excepted) after it shall have been presented to him, the Same shall be a Law, in like Manner as if he had signed it, unless the Congress by their Adjournment prevent its Return, in which Case it shall not be a Law.