Apple may have announced their ringtone strategy for the iPhone (30-second ringtones cost $1.98 to make and you must purchase songs through the iTunes Music Store), but Ambrosia Software's iToner utility lets you make ringtones from any mp3 or acc audio file with a simple drag/drop, all for $15 (free 30-day trial). iToner seems like the clear winner here.
Update: The just-released new version of iTunes (7.4) makes iToner ringtones invisible to the iPhone. Ambrosia is working on an iToner update. (thx, jim)