Many years ago, I read an article in Air & Space in which a bush pilot demonstrated the short-field capabilities of his airplane by taxiing out and then taking off across the width of the runway. What are the regulations regarding this sort of maneuver -- when ATC clears you to take off from a runway, are you required to take off down the runway, or is this merely customary?
If you're cleared for takeoff from runway 28, for example, you are expected to takeoff along the centerline of that runway. If you want to takeoff across the runway you should get an approval/clearance from ATC.
An ATC "clearance" for takeoff from runway 28 means that your magnetic course during takeoff must be the same as the magnetic alignment of the runway (approximately 280 degrees in my example - exact runway numbering is rounded to the nearest 10 degrees). You would not be in compliance with this clearance if you took off across (perpendicular) to this runway.
This is a pertinent Federal Aviation Regulation:
14 CFR91.123(a) states the following:
When an ATC clearance has been obtained, no pilot in command may deviate from that clearance unless an amended clearance is obtained, an emergency exists, or the deviation is in response to a traffic alert and collision avoidance system resolution advisory. However, except in Class A airspace, a pilot may cancel an IFR flight plan if the operation is being conducted in VFR weather conditions. When a pilot is uncertain of an ATC clearance, that pilot shall immediately request clarification from ATC.