I'm starting this as a new thread because I think there's some stuff in this that tangents away from the specific category of 5k v. 10k racing, however, it is in response to the discussion that started there. I've personally never, ever been a fan of even splitting. It might just be me, but my body continues to warm up until it has reached the point of exhaustion, at which point it drastically falls off the pace. Almost every race I've ever run (the marathon being the exception where I fell off pace about 20 secs/mile with 5 to go) I've always run negative split. This includes when I qualified for Nationals in the 800 meters in 1989. Even in this race I went out firm and comfortable through the first half, hit the 3rd quarter of the race hard and driving and then tried to keep that pace continually increasing through the finish. The result was 4 successive 200 meter negative splits. I think that a shorter race may demonstrate more poignantly the importance of negative splitting. An even paced race is run (if the intention is to run the best you possibly can on that day) essentially right on the bonk line the whole way. You have to know exactly where that line is for the distance you're running and then ensure that you keep pressed right up to the limit without going over. On the other hand, an intelligently run negative split race may give you a better chance to ensure that you press the limit without breaking yourself into a bonk mode too early, thus allowing you to deal with serious fatigue for a shorter period of time, even if it is potentially a higher level of fatigue. Put another way ... "A negative-split strategy utilizing heart rate and pace as tools for time-trial-type events, such as non-drafting triathlon, helps us manage the production and elimination of acid within our cells." In a shorter race run at significanly faster paces if a racer has a breakdown it becomes more readily visible. If you want to look at more of the details of this you can see them here
http://www.active.com/triathlon/Articles/Negative-split_strategies.htm. Here is a basic over-view on negative splitting and how to train for it
http://www.runnersworld.com/article/0,7120,s6-238-244--12106-0,00.html and here is an article that will help you understand negative splitting better by showing you how the Kenyans are often able to positive-split a race
http://www.pponline.co.uk/encyc/0837.htm -- note to most FRBers who live at altitude - you're living in an environment that may be helping your body be more equipped to deal with fatigue in such a way that with the right training you might be better prepared to positive split a race [this won't necessarily lead to a PR, however, it may actually interfere with your opponents racing technique in such a way as to tip the race in your favor over a potentially faster runner -- note, you'll have to change your training techniques]). I'd really like to hear from Sasha regarding this last article because I know he is often conscientious about neurological breakdown that occurs from too much intensity training. Note, this last article is rather scientific so if that's not your thing you might want to skip to the article summary.
All in all, it appears that your splitting technique depends on what you've trained for and a personal choice: Do you want to deal with a fatigue factor for a longer period of time at a steady stress level, or do you want to deal with fatigue for a shorter period of time at a more intense level.