The preamble from the 2009 Final 430 regs reads in part:
"These regulations clarify that the rules for short plan years apply for the year of termination by specifying that if a plan terminates before the last day of a plan year, then, for purposes of section 430, the plan is treated as having a short plan year that ends on the termination date. As a result, the minimum required contribution for such a plan is determined based on that short plan year. If a plan terminates before the date that would otherwise have been the valuation date for a plan year, then the valuation date for the plan year must be changed so that it falls within the short plan year. "
The actual language of the regs essentially repeats the above, as you would expect:
1.430(a)-1(b)(5) Terminated plans (i) Short plan year. If a plan’s termination date occurs during a plan year but before the last day of a plan year, then, for purposes of section 430, the plan is treated as having a short plan year that ends on the termination date.