Well, let me be the latest to join the Climbers with Asthma SP Support Group!
I had a severe asthma attack around age 12 (brought on by the stress of my parents' divorce) which then morphed into mild exercise-enduced asthma that was mostly aggravated by cold weather, which made my years on the high-school XC ski team extra fun. Not!
Fortunately, it's only bothered me a handful of times in adulthood and never in a climbing-related situation. However, just this past weekend, I went out for a trail run after not having run for about three weeks and felt like complete crap due to breathing trouble. The temps were in the mid 40's, which is much colder than I've run in recently, and I think some recent family/job/lifestyle stress may have also influenced it.
I'm glad to see that a lot of folks aren't slowed down by asthma, and am hopeful that it might actually help a little if I ever get to high altitudes.