Burgess J.W. Raby & William L. Raby have published Estate Tax Discounts on Retirement Plan Assets, also available on the Tax Analysts web site as Doc 2005-24154, 2005 TNT 230-26.  Here is the Introduction:

In Stock Valuations as a Matter of Law, Tax Notes, Aug. 31, 1998, p. 1059, we commented that "practitioners now have two well-reasoned cases saying that the hypothetical willing buyer should not be assumed to be willing to pay the same for two assets when there are tax detriments attached to one and not to the other." We then added, "While this may seem like common sense, it is only with these two cases that tax practitioners have a clear rejection of the IRS contention that tax discounts were simply not allowable as a matter of law regardless of the facts."

Assets other than corporate stock can have tax detriments attached to them, too, of course, and tax detriments can affect the willing seller. In fact, if the carryover basis regime comes into effect in 2010, there will be a wide range of assets that will carry a tax detriment to the sellers because they will have a tax basis to those sellers that will be different from their fair market value as perceived by the willing buyer. Does that mean the estate tax valuation of all those assets may be eligible for a tax discount? Put another way, does the effect of tax detriments on the price a willing seller might want to receive have the same effect on asset valuation as does the effect of tax detriments on the price a willing buyer might be willing to pay?