The policy on changing one's display name was changed back on February 4, 2010:
only one display name change is allowed every 30 days
However, there is no warning whatsoever when actually changing one's display name. Thus, it came as quite a bit of a shock to me when I found out today that I might be stuck as "Unicorns are Yummy" on Stack Overflow for the next freakin' month.
I would like to recommend that a warning be added to the "Edit Profile" page to warn people. Or, at least make the policy concerning display name changes more obvious.
I was able to change my display name back (phew!) by copying my profile from Meta Stack Overflow.
I still think a warning should be implemented, though.