This came up recently while discussing a Roth conversion. The folks in question had done a Roth conversion in 2009 and filed their 2009 taxes in April 2010. Now they're not sure that they meant to do that conversion in 2009 and are considering undoing it. (There may be a variety of reasons for undoing such a conversion, mostly beyond the scope of this note).
So the question posed was - okay - can it still be undone? How?
Normally, the deadline for an IRA re-characterization is the tax filing date for the tax year in which the transaction you're trying to undo took place. So in this case, since the conversion took place in 2009, the normal deadline would have been Apr 15, 2010.
However, there is an IRS regulation allowing one to take a 6-month extension on the deadline for certain elections - which include such a re-characterization.
The code section which authorizes this is 26 C.F.R. § 301.9100-2. The code may be read here: http://law.justia.com/us/cfr/title26/26-18.0.1.1.2.21.69.17.html
Fairmark wrote a nice article about this with more detail. It may be read here: http://www.fairmark.com/rothira/deadline.htm
In particular, assuming that the normal 1040 has been filed by April 15, they will need perform the re-characterization as discussed, and then correct their tax filing via filing a 1040-X (and possibly a new 8606). The updated filings need to be sent to the same IRS address as the original tax filing, and they need to write “FILED PURSUANT TO §301.9100–2” on the top of the document.