Not including HD-DVD in the console is the smartest move for MS.
Not including the HD-DVD drive and not having games support the format is MS telling us that HD-DVD might not come out on top and if it does win the format war they don't lose anything.
It is not politically correct for MS to come out and say that they are unsure if the HD-DVD format will win. As such including the drive as a add-on has given them (and us) the flexibility to back out of the format if Blu-Ray wins the war. I guarantee you that if Blu-Ray wins this war then we will see a Blu-Ray add-on for 360 and quite possibly a built in drive at a much later point. Also including games supporting the format. Vice versa if HD-DVD does win. A drive will come standard on the 360 and games will eventually support it.