I'm perfectly fine with phasing out Concentration. It was basically a one-trick pony of a skill that siphoned off valuable skill points from non-Wizards and non-Psions. Spellcasters can use Spellcraft to avoid having their spells fizzle when they take damage. Psionic characters can use Psicraft to do the same, and they should also use it to obtain Psionic Focus. Characters that take damage while performing skills that provoke attacks of opportunity should make a skill check with a DC equal to 10 + damage dealt to avoid having their work interrupted.
Finally, anyone using The Book of Nine Swords should change the Diamond Mind school so that it uses another skill as the base for its abilities (I suggest either Perception or Sense Motive). You may need to change the name of the school and the fluff text, but you'll end up coming out ahead with another great skill that you can use outside of combat.