Erica Sadun over at TUAW has found evidence in the latest iPhone firmware that Apple had built in table support for to-do tasks.
CREATE TABLE Task (ROWID INTEGER PRIMARY KEY AUTOINCREMENT, summary TEXT, priority INTEGER, due_date INTEGER, completion_date INTEGER, calendar_id INTEGER);
Erica notes that as of now, the support is left unused, as the firmware does not have the capability to sync or create to-dos. However, the find may indicate that Apple is planning an update to support the feature in the future.