[Jethomson] worked out a way to use a Nokia USB cable at a USB to Serial cable. He was able to pick up one of these cables for less than $3 delivered. A little probing worked out which conductors go with the appropriate signals and from there he developed a way to protect the 3.3v signal levels with a voltage divider.
It’s not surprising that this works, having seen [Will O'Brien's] post covering serial communications on Nokia phones. In that post we learned that the Nokia phones are using TTL communications. Once you’ve completed [Jethomson's] modifications to the cable you can follow his examples for using this in conjunction with an Arduino.