Are we going to be the ones to do it, or will the market do it?
The standard client is open source software. Just add a setting to display amounts in Bitcoins, Millies, Mikes or Satoshis.
If you're not a programmer, start up a bounty for a programmer to do this.
There's no possibility to change the definition of what "one Bitcoin" means. You can't have one name meaning different measurements. But using sub-units instead of the largest unit is unproblematic. Go for it!