Ultimately, Google intends for the transit planner to be part of its overall multi-modal trip planner (as in get from any point A on the globe to any point B – I hate the way they think small). In the nearer term we’ll see improvements like a better walking trip algorithm (to get you to the bus).
Interestingly, one of the thorny issues in trip planners has been to get transit agencies to agree on a common data model. It appears to me that Google’s entry into the field will prove catalytic, and their transit feed data specification will rapidly become a defacto standard. It’s relatively simple right now, but will become more robust over time as features are added.