The speculation, as I heard it few days ago, was that it was at a "state dinner" with a Russian diplomat that Yuschenko was poisoned, it is believed. I didn't hear all the details of the meeting or who it was with. Just that it was a Russian diplomat who hosted the dinner.
For example the rat poison warfarin (a blood thinner) is almost undetectable and some say it was used to bump off Stalin and make it look like a stroke.
It would seem unlikely that Russia would be so clumsy in hosting a dinner and then at that dinner use such an easily detectable chemical to kill its guest Viktor Yushchenko.
PS: The reason dioxin was not detected until now was that Yushchenko refused a biopsy. The sample would have been taken from his face and he did not want to do it for cosmetic reasons (stitches and bandaid cover on face) That also is a little odd seeing as how he had little to protect cosmeticaly speaking because of the rash.