Like everyone, I do most of my running near where I live. For me, that means that almost all of my training is at or above 10,000ft. Of the 1,700 miles I ran in 2011 I bet only 100 miles were below 7,000ft. And probably only 300 miles were below 9,000ft. That's both good and bad. Training at altitude has its advantages, but Leadville is at the extreme. The effects of altitude are not linear-- atmospheric pressure decreases exponentially with altitude.
I remember when I first moved to Leadville in 2006 I was acutely aware of the elevation of my new hometown. I got winded simply unpacking our moving truck. The grocery store aisles were full of bags of chips that looked like balloons ready to burst. Baking anything was an adventure and water boiled at a mere 193F. (I'm a home brewer and I eventually discovered that the lower boiling point of water at 10,000ft was playing havoc with my hop utilization. Strangely, my first few batches of IPA were not as hoppy as they should have been. After some research-- and much trial-and-error-- I found that I had to significantly raise the amount of bittering hops I was adding to the wort to compensate.)
Becoming a father brought with it a new lesson in high altitude living. Most newborns in Leadville require supplemental oxygen for the first few months of life until their bodies adapt to the crazy elevation at which their parents decided to live. My daughter was no exception. I can attest to the fact that learning to change a diaper while there's 25ft of oxygen tubing attached to your daughter's face presents a challenge! (As it turns out, requiring oxygen is much rarer for newborns in Summit County-- which is just 1,000ft lower than Leadville-- evidence that the effects of altitude are not linear.)
Running at 10,000ft means that most of my runs-- especially the uphills-- are limited by my aerobic capacity rather than my leg strength. In other words, I'm generally gasping for breath. Since I've only been running for two years, running in Leadville is pretty much all I've known. It's normal. Whenever I travel to sea level I'm always curious to see if I can detect any differences in my performance. My resting heart rate drops about 10 bpm at sea level. And when I glance at my GPS while I'm running I'm often running about 1 min/mi faster than I'd expect. What I'd guess was a 9:00 min/mi effort turns out to be more like a 8:00 min/mi pace at sea level. Also, gentle uphills and rollers seem to have less effect on my pace than comparable terrain in Leadville. These changes aren't earth shattering-- it's not like I'm suddenly going to start winning races at sea level-- but they are noticeable. On the flip side, the other thing I notice at sea level is that I have no idea how to run faster than 7:00 min/mi! My legs have so little experience moving that fast since I can only sustain that pace for a mile or so on a few select routes in Leadville. The conditions need to be perfect: a smooth surface with good footing and, of course, a gentle downhill grade. If I trained at a lower elevation I could regularly train at a higher intensity-- and it's possible that the benefits of higher-intensity training could cancel out (and possibly surpass) any of the advantages I may have due to simply living at 10,000ft.
In my opinion, the elevation of Leadville 100 course cannot be ignored, but its effects are probably overstated-- especially for those racers who live anywhere in Colorado. Yes, ideally, if you're coming from sea level you probably want to arrive a few weeks before the race and climb some 14ers to acclimate. And, yes, you'll probably run a bit slower than you normally do. But for most runners the race is going to be a 25+ hour affair and there are so many potential reasons you could end up running slower-- poor hydration, poor nutrition, injury, sleep deprivation, exhaustion, etc.-- altitude is just one variable among many. When all is said and done, living in "Cloud City" is an advantage when it comes to running the local 100, but it does not guarantee success. And whatever advantage living in Leadville bestows, it may have as much to do with its supportive and knowledgeable community of runners, and its easy access to mountain trails for training, than it does with its altitude.