Can the iPhone use other databases besides SQLite, like MySQL?
The iPhone can only use SQLite as a database, directly on the device. This means there is no MySQL server inside the iPhone. But you can, and are free to have (your own) MySQL Back-end server, to which iPhone applications connect to. But Apple doesn't provide the server, so you have to pay the maintenance costs
SQLite is installed on all iPhones. Theoretically, you can run other databases but since you cannot start other processes, you cannot run MySQL -- MySQL requires a separate process (the mysqld daemon).
Not true any more!
The Raima RDM Embedded database SDK will also work on the iPhone. It's cool because it supports the network and relational models, and you can do things like having direct pointers to records for faster access.
They have an official port coming, but their version for Mac works in the iPhone Simulator.
http://ODBCrouter.com/ipad (new) has XCode client-side ODBC libraries, header files and multi-threaded Objective C objects that let your apps interact with just about any database server using industry standard ODBC programming calls. The server-side component runs on a Windows box (or VM), but the databases themselves can run on Mac OS X, Windows, Linux, IBM iSeries or AS/400, Unix, NetWare, DOS and legacy mainframes with support for most revisions of MySQL, 4D, QuickBooks Pro, DB/2, MYOB, Empress, Oracle, SQL Server, MS Access, Excel, Navision, Firebird, Pervasive, Sybase, Pick, Universe, Informix, Ingres and SQLite. You can see some screenshots of it in action at the link.