Well, because there is no gas contract for 2006 between Naftogas and Gazrprom and this one is commercial contract, whereas fleet agreement is a package international treaty which includes Russia's recognition of Ukraine's borders.
There is no Turkmen gas for the first quarter of 2006 for Ukraine: this gas is all bought out by Gazprom.
I think that just about kills the optimistic scenario.
The Russians are going for the jugular, here. Have Keyboard. Will Travel. :)