A qualified retirement plan is sponsored by a sole proprietor. The plan solely covers the owner-employee and is therefore not subject to Title I of ERISA. Was the plan required to adopt an amendment for compliance with the automatic rollover provisions under IRC 401(a)(31)(B)?

That's an IRC requirement, independent of ERISA. So, Yes.

But, correct me if I'm wrong, it's only needed if the plan has a deminimis cashout limit greater than $1000.

Notice 2005-5 might be of some use: http://www.irs.gov/pub/irs-drop/n-05-05.pdf

You're correct, Masteff, but that would encompass the vast majority of plans prior to the change to Section 401(a)(31)--unless, of course, the plan contained no involutary cash-out provision at all, in which case no amendment would be necessary.