Say a taxpayer has a plan year that ends on August 31, 2005. What serves as the latest date to amend for 401(a)(31)(B)?

http://www.irs.gov/irb/2005-51_IRB/ar12.html

Latest of (1) December 31, 2005, (2) the end of the plan year that contains March 28, 2005, or (3) the tax filing deadline for the employer’s tax year containing March 28, 2005.

That seemds to indicate that December 31, 2005 would serve as the latest possible day to amend the plan, as the play year containing March 28, 2005 ends on August 31, 2005; meanwhile, the tax deadline would fall on November 15, 2005.

Do I understand this properly?

No, because the "due date for the tax return for the taxable year containing March 28, 2005" includes extensions of that due date (as specified in EPCRS & Notice 2005-95). So, I see the absolute deadline for amending for IRC Section 401(a)(31)(B)--assuming the taxpayer obtained an extension for filing the tax return for the year ending August 31, 2005--as May 15, 2006.