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."
This finding matches the best practices I advocate in Web Form Design:
"Literally including the phrase “optional” after a label is much clearer than any visual symbol you could use to mean the same thing. Someone may always wonder 'what does this asterisk mean?' and have to go hunting for a legend that explains things."