The pitcher must visually indicate to the umpire, batter and runner(s) which way he will begin pitching to the batter. Engaging the rubber with the glove on a particular hand is considered a definitive commitment to which arm he will throw with. The batter will then choose which side of the plate he will bat from.
The pitcher must throw one pitch to the batter before any "switch" by either player is allowed.
After one pitch is thrown, the pitcher and batter may each change positions one time per at-bat. For example, if the pitcher changes from right-handed to left-handed and the batter then changes batter's boxes, each player must remain that way for the duration of that at-bat (unless the offensive team substitutes a pinch hitter, and then each player may again "switch" one time).
Any switch (by either the pitcher or the batter) must be clearly indicated to the umpire.
There will be no warm-up pitches during the change of arms.
If an injury occurs the pitcher may change arms but not use that arm again during the remainder of the game.
Let the record reflect that I blogged about Pat Venditte back in April.