Something ancient that everybody knows. Usage originated with the sarcastic statement "Corn laws repealed!", a variant of "Earth comes into being: Big Bang suspected". Used mainly by tossers who think they're at the cutting edge of everything, or by people wary of being made to look less-than-hip by aforementioned tossers.
"Apologies if this is corn, but has anyone heard the latest David Beckham rumour?" "Jesus fuck, that's so corn!" etc.