I do not like it. If that person asks if he can draft, that's one thing, but to just silently do it? You are not welcome on my wheel. Why? You are asking me to ride with you in mind and putting me at risk if I don't. I do find it sometimes amusing that folks will do this when I am riding fixed. I wonder if they have any idea how fast I can take them out if I get surprised (say by someone stepping into the street).
Minor point: Oregon law specifically prohibits drafting. Bicycles are, by law, vehicles and are subject to the same laws except where noted explicitly. Nowhere is tailgating excepted.
Ben