Battle Caster

Obsidian has confirmed the existence of the Battle Caster feat.

DnD 3.5 Battle Caster
Type: General

Prerequisites: Warlock or Bard

Benefit: You may wear armor one category heavier than you could normally wear while avoiding Arcane spell failure checks. This allows Bards and Warlocks to wear medium armor without risk of arcane spell failure.

Special: This feat requires a class that in some way already mitigated Arcane spell failure from armor. Thus, it won't benefit Wizards or Sorcerers as they are penalized by all armor.