the rate of heat loss from coffee to air is directly proportional to the difference in temperature between the coffee and the air. thus black coffee will have a higher rate of heat loss than coffee that has been slightly chilled by the addition of cream.
assuming the cream is to be kept at a constant temperature until it is added to the coffee whether it is added immediately or just before drinking, you're better off adding it to the hotter coffee.
however, if the cream is to be exposed to room temperature air while waiting to be added it's a wash. if you add the cream right away, the coffee won't lose heat as fast initially. if you add the cream later, it will have warmed slightly and won't cool the coffee as much.
at least that would be true if the cream didn't have a thickening effect on the coffee. more viscous fluids lose heat more slowly to their environment since the distribution of heat within the liquid has a larger gradient. basically, coffee with cream can be slightly colder at the top before being displaced by warmer coffee/cream below. with this consideration i'd say you're always better off adding the cream as soon as possible.
also,
http://lifehacker.com/software/stuff-we-like/usb-coffee-warmer-149297.php
