Earlier this week Apple announced they'll be selling customizable ringtones for iPhones - but that users will have to first purchase a song for 99 cents and then pay another 99 cents for the right to use the track as a ringtone. A clever boy by the name of "cleverboy" over on Macrumors forums has already figured out how to use Apple's own software to turn AAC music files into iPhone-compatible ringtones.
The forum thread also contains posts referring to iToner, a $15 software application from Ambrosia, that allows for custom iPhone ringtone creation. iToner is a bit more user-friendly than Cleverboy's method, but both roads seem to lead to whatever song you want blaring out of your iPhone's speakers every time you get a call.
Note that these methods may not be entirely compatible with tracks purchased from the iTunes store. iTMS files are protected by digital rights management technology meant to prevent "unauthorized use" of audio content - like creating ringtones from files without paying Apple (who, in turn, pays the record labels).