If you have never heard of Jason Gambert before, you soon will. He is creating a storm in the US by trying to register SEO as a Trade Mark.
The laughable side of this furor is that he claims to have coined the phrase in 2007. Naturally, the SEO world is up in arms and SEOMOZ has a very informative post, which is worthy of a read in full.
Anyway, a quick search of my email database reveals usage of SEO back in 2003 so I am in with a shout, surely?!