Mikel Maron hacked the iPhone Maps application to allow him to put in different tiles, in this case those of OpenStreetMap.

He details replacing the sqlitedb and discusses the SQLite schema:

sqlite> .schema
CREATE TABLE images(zoom int, x int, y int, flags int, length int, DATA blob);
CREATE TABLE version(version int);
CREATE INDEX index1 ON images (zoom,x,y,flags);

NOTE: I have no idea about the legality of this hack