For the past half year, I have taken to dressing on the warm side - an extra fleece layer during xc ski season and continuing to wear my xc ski jacket and pants in runs now moving into spring. I'm doing it mainly to experiment after reading Benji Durden's comments here a few months ago (that he tried both but always raced faster when his training was done in sweats). I used to do the opposite: dress as lightly as I could get away with in the cold... I know that the college xc team here trains in sweats for heat training purposes. The temperatures are cool to cold in the fall here and they wouldn't otherwise be able to run well when competing in meets in CA for example.
Your body does acclimate to heat. I did a 2.5+ hr run yesterday in windpants, t-shirt and a Craft xc ski jacket (shell with microfleece lining and vented back) in temperatures near 70F and I was quite comfortable - a nice light sweat on my face, my cotton T-shirt was pretty much dry when I finished, and I only drank one small bike bottle of water during the run. Without acclimation, I'm sure I would have been REALLY hot wearing what I was wearing. I have yet to race this year, and I'm not expecting amazing jumps in performance in ideal cool conditions, but races are held in warm conditions too and people race in them. I didn't read the report on this year's Boston marathon, but I remember all the excuses for running poor in the heat last year. They were certainly valid excuses... they weren't ready for the heat training in cool temperatures but racing in hot temperatures. But, if they had trained in sweats (like the local college team here does...), they could have been ready for the heat.
As for dealing with the cold if you are always training warm, I say that you can always dress warmer. There is no reason to run slow in 20-30F... that's SUPER comfortable for xc skiing because they have on enough clothing. The Kenyans (and others) who run poorly in the cold shouldn't try to tough it out in shorts and singlet...