Lua aims more at readability than conciseness, yet does an honest 37 chars:
function p(s)return s==s:reverse()end
variant, just for fun (same size):
p=function(s)return s==s:reverse''end
The JavaScript version is more verbose (55 chars), because it doesn't has a string reverse function:
function p(s){return s==s.split('').reverse().join('')}
