As he says. Little known but not at all hidden fact: 99% of ALL internet culture comes from 4chan. Every fucking little clever meme you like and all of those clever viral videos all spawn from that cesspool. It is really quite interesting to watch. Eventually 4chan's influence even reaches those outside of the interwebz - you start to see people on fucking fb(not interwebz territory per say) typing in 4chan slang(and almost nobody even knows they are using their slang). Hell how many people now refer to bad attempts at what-the-fuck-ever as "fail"? Where do you think that came from? lol... 4chan is so ridiculously influential on our culture...
In short "teh" is a spelling derived from 4-chan slang. It has seen use throughout the interwebz since its inception.
I would however argue wikipedia on it being at all related to l33t as I know firsthand it is not. L33t 15 R353RV3D 4 |\/|45k1N6 5P33CH 1N a N4TUR3 51MiL4R 70 7h15. This is elite or "1337" speak.
I said "Leet is reserved for masking speech in a nature similar to this." by the way