I am assuming (I think correctly) that the full distribution of 401(k) assets took place entirely in 2009.

With that being said, you put your 1/2 of the 401(k) account value (as of the Distribution date) on line 16a of your Form 1040 for 2009.

Probably the best way to explain it is to look at the situation from your Ex-spouse's point of view (I understand if you don't want to, but please bear with me):

Your Ex will need to pay Taxes on the half from your 401(k) (on Line 16b);

Your Ex also needs to list this amount on Line 16a (since 16b canNOT be less than 16a);

That leaves your half of the 401(k) to place on your Line 16a (since both Lines 16a must not be greater than the total of the 401(k) Distribution), and;

Up to your Line 16a amount will end up on your Line 16b (for you to pay Taxes on).

As a side note, in the court case Clawson v. (Tax) Commissioner, it was ruled that the Ex-spouse cannot make the other ex-spouse pay the taxes on the entire Retirement Plan Distribution. In other words, each ex-spouse must pay Taxes on the separate Retirement Plan Distributions received, no matter whose Retirement Plan the whole thing used to be.