I had fingered Lugovoi as one of the prime suspects. See the archived posts for December 03 - 10, 2006. My suspicion became most focused on Lugovoi as a result of the item of evidence mentioned in the 14th comment to the blog post of Friday, December 8, 2006. See also, at the same link, the earlier blog post of December 6, 2006, Bungler-Murder Hypothesis 2.1.
I had suggested that time lines might help sort out the evidence and the possibilties. I see that the New York Times and the AP agree with my sentiment:
Timeline in Poisoning of Ex - KGB Agent, New York Times (May 22, 2007) (AP).
Of course, as Priit Parmakson has pointed out, the New York Times has been enamored of time lines for quite some time. When I teach my course in fact investigation next (in the fall of 2008) I must point this out to my students.
If this case ever results in a public trial -- but, alas, it appears this is unlikely -- students of evidence, inference, and investigation would have great clay to sculpt.