For those of you riding kilos or 500m TT's, there's been a rule change as of Feb this year that you may have missed. It is now illegal (unless you have a morphological exemption) to use aero bars/clip ons for the 500 and kilo AND be closer than 50mm to the bottom bracket. Sprint events (sprint, team sprint, kilo, 500, keirin) allow you to position your saddle up to 0mm behind the bottom bracket and this, until February, was ok including the use of aerobars, :

1.3.013 The peak of the saddle shall be a minimum of 5 cm to the rear of a vertical plane passing through the bottom bracket spindle. This restriction shall not be applied to the bicycle ridden by a rider in a sprint event on track (flying 200 m, flying lap, sprint, team sprint, keirin, 500 metres and 1 kilometre); however, in no circumstances shall the peak of the saddle extend in front of a vertical line passing through the bottom bracket spindle.

but rule 1.3.023 overrides this rule :

1.3.023 For road time trials and individual and team pursuit on the track, a fixed extension may be added to the steering system; in this instance, only a position where the forearms are in the horizontal plane is permitted. It is also possible to add a handlebar extension for the 500 m and kilometre time trials on the track, but in this case, the position of the tip of the saddle must be at least 5 cm behind the vertical plane passing through the bottom bracket axle.

This is effective as of Feb 2013. If you have an older copy of the rules, 1.3.023 HAS CHANGED. Anyone training for kilos or 500's needs to be aware of this rule change if you're intending to use aero bars. I expect (but am not certain) that the same applies to masters riding 750m ITT's.

I hope this helps clear up a little confusion about the matter.

The rule is here :

http://www.uci.ch/includes/asp/getTarget.asp?type=FILE&id=34033