Default d20 falling rules suck hard. I won't bother explaining why, they just do. Here's my revision...
Base falling damage is 1d6 per 10 feet fallen, capped at 60d6 (usually at 600 feet fallen). For purposes of damage resistance, this is considered Bludgeoning damage.
- If you are not flat-flat-footed when you began falling, you make make a Reflex/Jump/Tumble check (choose your best one). The DC is equal to 5 per die of base damage. Success completely negates one die of damage. If you are falling/diving into water that is not deep enough to attempt a dive (described below), you can substitute a Swim check with the DC outlined here.
- If you are not flat-footed when you began falling, and you are falling into water, you may attempt a Swim check to dive into the water. This check is instead of the Reflex/Jump/Tumble check described above. The DC is 10 (+1 per die of base damage). Success completely negates one die of damage. Each full five points the DC number is beaten by negates an additional die of damage. The water depth must be at least equal to the distance fallen or 30 feet (whichever is lower) in order to attempt a Swim check. If the water is not deep enough to perform a dive, you can instead attempt a controlled "fall" as described in the bullet point above, and use your Swim skill.
- Falling onto a soft surface (deep mud, deep snow, cart full of straw, etc) negates an additional die of damage. Note that water is not considered a soft surface for this purpose.
- If you fall onto a spiked pit, this is treated as a regular fall. The spikes then perform an "attack", with 1d4 attacks, bab +10, and 1d4+4 damage per attack (these numbers are based on the Traps section of the SRD).
Roll a Fort save (DC 15 +1 per die of base damage). Failure means you are stunned for one round when you land. Each point that this save DC is beaten by reduces the Dexterity damage by one point. Making the DC number exactly means you are neither stunned nor is Dexterity damage reduced.
So How Do Folk Survive Falling from Orbit or Aircraft?
If you have any action points to spare, you can spend one to cap hit point and Dexterity damage to "all but the last point".
Special; Sci-Fi Considerations
Air Pressure: The 60d6 damage cap is not affected for air pressures lower than Earth's. For higher air pressures, multiply the damage cap by (1 / cube root of air pressure). For example, Venus (95 atmospheres) would have falling damage capped at 13d6. Of course, you'll be simultaneously crushed, suffocated, dissolved, and roasted, once you land there...
Gravity: The effective distance fallen is multiplied by local gravity; a 60-foot fall on Earth's Moon (1/6 Earth-G) would be treated the same as a 10-foot fall on Earth. This can get dangerous fast on planets with high gravity. (Note that current theories of planet formation don't allow for rocky planets with surface gravities over 3 G; such bodies tend to sweep surrounding space of light elements, and become gas giants).
Air pressure and gravity considerations can be pre-calculated when the planet is designed, and left as a single sentence summary described in the planetary environment chapter.
Venus: 1d6 damage per 10 feet, capped at 13d6.
Earth: 1d6 damage per 10 feet, capped at 60d6.
Mars: 1d6 damage per 30 feet, capped at 60d6.
Luna: 1d6 damage per 60 feet, capped at 60d6.
Orbital Re-Entry: Thermal effects of orbital re-entry coming soon.