On the Interaction Design Association (IxDA) discussion list, Erin Walsh shared some interesting insights on marking Required vs. Optional form fields.

"We recently did prototype testing on several search forms with a mixture of required and optional fields. On the team we were split on the best approach, so we tried to distinct methods: one with optional spelled out, the other with those fields having a different visual indicator. Though the sample size was limited, the "Optional" won hands-down. Remarkably, some participants even commented on how much they liked that it said "optional right there". I know our UI team was not thrilled, but it was extremely advantageous to spell it out rather than use an icon or other visual indicators."

-Marking Required vs. Optional form fields, IxDA Discussion list