I figured I may as well post the current setting rules for Slalom and GS since the discussion has gone a bit in that direction..Slalom
For Slalom, the slalom the distances are measured from "pin-to-pin", or from turning pole to turning pole.
Max distance: 13m
Flush / Hairpins: 4-6m max
Distance between "doubles" in Flush / Hairpins: 75cm (min)Giant Slalom
For GS the setting is done based on a percentage of the vertical drop from the start to the finish. The number of direction changes must be equal to 11-15% of the vertical drop. The minimum distance between any two gates is 10m.
Therefore the distance between each gate can vary greatly depending on the hill. However, for the most part at the FIS level and above, the setting is in the 26-32m range.
All the above rules can be viewed at www.fis-ski.com
or here (most recent blue book):http://www.fis-ski.com/data/document/ICR04.pdf
Don't forget that the blue book isn't the complete list of rules, you must always check the precisions to see if anything's been added or changed.
One other thing to add is not to forget that most GS skis have radii much greater than 21m. Although some of them are stamped 21m, most are actually stamped >21m, which of course means "greater than" 21m. This is because the 21m ski is actually the shortest of the lengths available for that ski (175 for most FIS legal skis), so as the length increases so does the sidecut radius. Most 190ish GS skis will have radii in the 27m range.