Although Apple’s documentation doesn’t make too much of the point, the iPhone offers full support for SQLite. If you’re comfortable with DBs, this can be a timesaver, even for some problems for which a DB might seem like overkill. (In iPhone OS 3.0 and up, Core Data is also supported.)
This post offers a good introduction to the topic. Pay careful attention to the discussion of the libsqlite3.0.dylib file, as it’s the only really non-obvious part of getting started.
Incidentally, is it just me, or are there a disproportionate number of Aussie iPhone developers/resources?
