Can anyone direct me to any written guidance, either formal or informal, that requires a frozen plan to be funded on a unit credit basis? Thank you.

2004 IRS Gray Book, question 20. Gray Books are distributed at the annual Enrolled Acutary's Meeting. IRS states emphatically that frozen plans are required to change to PUC method. (Note, use the PUC method, and not the UC method. The automatic approval of Rev. Proc 2000-40 is for the PUC method. Results should be the same unless the Plan formula is compensation based and freeze is soft and allows average comp in respect of service as of the freeze to increase.) Of course, this is the gray book and represents thinking of IRS participants in "think" session and does not represent official guidance.

The IRS view ultimately relates to "what is a reasonable funding method?"
Read here some prior discusions:
http://benefitslink.com/boards/index.php?showtopic=29346

Your question suggests that you are doing a 2006 or 2007 year.

While I agree that IRS expected PUC funding method before 2006, that discussion is now obsolete. We have had versions of PPA discussed since at least November 2005, and Congress specifically went with the TUC method in their changes to funding rules. PPA is now the law, starting in 2008, and it serves as a powerful argument for years 2006 and 2007, if you want to use it. From your question, it sounds like you don't want to use it. Any reason why?

Since you can fund for the full current liability now, the range of contributions is very wide and flexible. With the 50% cushion, or PUC accrued liability cushion, you can still get higher contributions than TUC on PPA rules.

2000-40 did make it clear that the IRS expects you to assign all accrued benefits to the past for a frozen plan.
This effectively meant 5-10 year funding for remaining unfunded benefits. A "soft-freeze" design is not something I use, so using PUC is a decision that seems irrelevant to me.

Andy,
It's a hard freeze, so PUC becomes TUC.

SoCal,
You're right, it's a 2007 val. I want to make the change, but the sponsor is now balking at the increased contribution requirement. I wanted some backup for my position. In any event, as you point out, even if they don't go for it for 07, they'll have to for 08.

Thank you for all the replies and links.