When others attack (with spells and magic weapons or such abilities) you while you are incorporeal, you have a 50% chance to ignore the damage. When you attack while incorporeal, you also have a 50% chance of not harming your opponent (while attacking with a spell or magic weapon).
Are ghost touch weapons the only way to bypass that drawback?
The d20srd.org entry on Incorporeal Creatures has the answers you seek:
There is no 50% miss chance for Incorporeal creatures hitting other creatures unless the incorporeal creature is within a solid body (and thus can't see its target, granting it total concealment. An incorporeal creature's attacks ignore armor, natural armor and shield bonuses so they can be quite accurate against many foes!
The following ways can damage an incorporeal creature without having to worry about the 50% miss chance:
Transdimensional Spell is a feat that was originally in Unapproachable East (p. 45) and now is in Complete Arcane (p. 84) and Complete Divine (p. 85). It allows one to cast spells that affect incorporeal creatures as though they were corporeal.
There are also spells and effects from Ghostwalk that can drag incorporeal creatures into manifested corporeal bodies.
My answer is divided in 2 parts
1 PART) When a corporeal creature wants to attack an incorporeal
The incorporeal subtipe from "Rule compendium" 64 states the following:
An incorporeal creature can be harmed only by other incorporeal creatures, magic weapons or creatures that strike as magic weapons, and spells, spell-like abilities, or supernatural abilities. Even when hit by spells (including touch spells) or magic weapons, an incorporeal creature has a 50% chance to ignore any damage from a corporeal source
continues
except for positive energy, negative energy, force effects such as magic missile, or attacks made with ghost touch weapons.
So by description of incorporeal subtype the following are ways to ignore the 50% miss chance:
By Type of Spells: Incorporeals are affected without miss chance by enchants:
By Weapons
By Feat
A transdimensional spell has full, normal effect upon incorporeal creatures, creatures on the Ethereal Plane or Plane of Shadow, and creatures within an extradimensional space in the spell’s area
in meelee, everytime you miss because of a creatures incrporeal's miss chance, you can reroll your miss chance percentile roll one time to see if you actually hit
This is applied if you are attacking an enemy that became incorporeal through a spell.
Your fierce contempt for magic allows you to disregard the miss chance granted by spells or spell-like abilities such as darkness, blur, invisibility, obscuring mist, ghostform (see page 109), and spells when used to create concealment effects (such as a wizard using permanent image to fill a corridor with illusory fire and smoke). In addition, when facing a creature protected by mirror image, you can immediately pick out the real creature from its figments. Your ability to ignore the miss chance granted by magical concealment doesn't grant you any ability to ignore nonmagical concealment (so you would still have a 20% miss chance against an invisible creature hiding in fog, for example).
By the strategy of becoming yourself incorporeal why? - in the "rule compendium" (64) it is not mentioned that incorporeals have any miss chance of attacking other incorporeals.
Becoming incorporeal by a spell some examples are as follows:
By class feature:
Becoming a ghost Becoming Ethereal (Rule Compendium page 41) does not mean becoming incorporeal as only ghosts have the power to manifest (while being in the etheral plane they manifest as incorporeals in the material plane). This is also discussed in a previous question.
So if you make yourself becoming a ghost you could use the same strategy.
By the strategy of making the incorporeal corporeal with a spell
2 PART) When an incorporeal creature wants to attack an corporeal
By description of incorporeal subtype above (Rules compendium page 64) the following are ways to ignore the 50% miss chance:
Use a spell WHY? - Because spells have ha 50% miss chance if casted by a corporeal to attack an incorporeal, but incorporeal creatures casting a spell on corporeals have no miss chance. (not symmetric rule)
** By Feat**
By Weapons
Becoming corporeal by a casting a spell on yourself some examples are as follows:
By Feat Ghostly grasp from liber Mortis. Which allows you to touch corporeal things.
Becoming permanently corporeal this is discussed here
Limits of this answer: I tried to include as many information as I was able to collect and review but I am sure I have missed something, thus I will edit this answer if I recieve any comment or find new information.
Also, in regards to Pat Ludwig's answer, the rules of incorporeals where updated from the Monster manual 1. Some of the handbooks where you find the updated rules of the incorporeal subtype rules are as follows: Monster Manual III page 214 Rule compendium page 64 Tome of Battle page 79
