You may be right about the uncertainty about the medieval warming period. There does appear to be a lot of conflicting information. On the other hand whether Greenland was warmer then than it is now is not really relevant, as we are talking about global rather than local effects.
There is also a lot of controversy about the "hockey stick" graph, although this guy from New Scientist seems to think it is much ado about nothing.
Regarding the location of the weather stations, they use data from rural and air balloon based stations to 'calibrate' the urban ones. I have read that the estimated effect of the urban heat bubble was something like a hundredth of a degree.
