In the panel discussion I participated in Tuesday (see post below), the issues of pseudepigraphy and pseudonymity were raised by one of the panelists. Since this is an important issue that challenges the inspiration, authority, and inerrancy of Scripture, I thought I would post a few thoughts.
1. First, some definitions. Pseudepigraphy ("false superscription") refers to writings that have been falsely attributed to a well-known person. Pseudonymity ("false name") is used synonymously to refer to the same phenomena, though as Carson and Moo point out, only the latter term can be traced back into antiquity. Examples include works like Wisdom of Solomon, 3 Corinthians, Assumption of Moses, Testament of Job, etc.
2. This phenomena encompasses a variety of motives, ranging from outright attempt to deceive to mistaken conclusions by well-meaning people. In other words, some authors intentionally claimed their work was that of someone else to deceive the audience and claim the authority of the falsely named author. At the same time, other works over time came to be associated with a figure with no intention to deceive; these were "honest" mistakes.
3. A distinction must be made between those works that are anonymous and later came to be associated with someone and those that make explicit claims to authorship. For example, the work called "Wisdom of Solomon" never explicitly claims to be written by Solomon (though 7:1-14 & 8:17-9:18 strongly suggest it); by contrast 1 Enoch directly claims to come from Enoch himself. This distinction is important when we come to the NT. It is one thing to note that Hebrews was (wrongly) thought by some in the early church to be written by Paul (it is anonymous); it is quite another to say that Ephesians was not written by Paul (despite its explicit claim).
4. Despite the fact that this was a common practice in the ancient world, there is absolutely no evidence that the early church ever knowingly accepted a pseudonymous document as authoritative. Again the discussion of Carson and Moo is instructive; they point out that even works (such as 3 Corinthians) that were highly regarded in parts of the early church were condemned when it was recognized to be falsely written in the name of Paul.
5. Therefore if any of the documents in the NT are in fact pseudonymous, they were accepted unknowingly. Furthermore, given the dating that many scholars give to "pseudonymous" letters such as Ephesians and the Pastoral Epistles (usually late 1st century, well after Paul's death), one must conclude that the recipients knew they were receiving a letter falsely written in the name of Paul. But then at some point this "knowledge" was lost. How does that happen?
6. The direct statements about pseudonymity in 2 Thess 2:1-2; 3:17 are often not fully appreciated. Paul explicitly warns the Thessalonians about being shaken by a letter claiming to be from him, and then concludes by noting that the writing of the postscript in his own hand was a distinguishing mark of his letters. Paul explicitly condemns the writing of a letter in his name. Of course, many critical scholars claim that 2 Thess is itself pseudonymous, which would mean that the real author of the letter was condemning the very practice he was engaging in! Talk about hubris!
7. Claims of pseudonymity, therefore, are usually based largely (if not entirely) on internal matters such as vocabulary, style, theology, etc. But notice how subjective such claims are! Do we really have enough of a body of writing from even Paul to emphatically state that Paul could not have written in a certain way? What about the potential role that a difference in amanuensis (secretary) would make in vocabulary and style. What about the difference in historical circumstances that Paul addresses; wouldn't they make some difference in vocabulary, style, and theology?
8. At the end of the day, claims of pseudonymity are a direct denial of the inerrancy and inspiration of Scripture, despite what some may claim. While as evangelicals we must not shy away from critical examination of the NT documents, we must also reject the naturalistic assumptions that frequently drive claims of pseudonymity.
This is but a brief excursion into the subject. If you want further discussion, let me once more direct you to the discussion in of Carson and Moo, pp. 337-350.