Do you mean microsoft SQL server, or some other RDBMS that uses SQL? If the former, I would try to find a library that alllows ODBC access in C. You can call into this library from Objective-C, and make a wrapper class around its functions if desired. Note that I have not had to solve this myself, so these are just my suggestions based on the approach I would take.
-Lee
Edit: too slow. If you want to be able to move between databases I would definitely write an interface that abstracts the DB, then implement that once per DB you need to target. This way no app code will need to change, since it will just use interface methods.