JiffyPopTart
Bree-Yark
It has been laid out earlier in this thread how Subtle Spell differs from just speaking at a low volume and slightly gesturing (like Obi-Wan Kenobi in my post).I think it must be a house rule, on the grounds that if it is not, then Subtle Spell has no meaning. That is, if you can be not noticed anyway, it is meaningless to have a specific metamagic that allows you to not be noticed.
Subtle Spell lets you cast a spell without any speaking at all, and without any hand gestures at all. This is very different and can be used for many benefits, only some of which intersect with being able to Obi-Wan Kenobi cast.
It let you do the following...
Cast a spell when gagged.
Cast a spell when your hands are tied.
Cast a spell when submerged.
Cast a spell when Silenced.
Cast a spell when polymorphed into a form that has no mouth or hands.
Cast a spell when even the slightest bit of noise is too much (when hiding 3 feet away from someone alone in a room).
Cast a spell unnoticed when being directly observed by someone watching for your to cast a spell.
....none of which you can do without Subtle Spell or some other special ability.