Ever since the 4.0.4 update my custom ringtones only play half the time. This is kinda bad, because I rely on them to let me know whether or not the person calling is important. But I haven't seen anyone else mention it, am I the only one? If I had to guess, I'd say that something changed in the update uses the "SD card" moreso than usual and when the call comes in, renders the audio files in question temporarily useless.
Not sure if it's the same issue that you are refering too, but I have a custom default ringtone for when everyone calls and then my mom and my girlfriend each have there own ringtone. Randomly when one of them call it will not play their specific ringtone and play the custom default ringtone i set for any call i get. Don't know how long it has been happening.