There was some discussion recently on the mailing list about this.  The general consensus was that:

(1) This needs to be done carefully to avoid people sending 1000x what they intended
(2) It is difficult to change an established system.  So we want to do it once.
(3) It'd be nice to only have two digits after the decimal since most accounting software can't handle more

Therefore, we go straight to micro-BTC.  It solves all of this in one fell swoop.  At the current Bitcoin price (~$1,000) that would be 1,000 uBTC per $1 USD.  That's a totally reasonable, human-usable system.  And accommodates all future non-hardforking growth (i.e. if we never need more than 8 decimal places, this will be last forever).