I doubt this is true, but a someone who goes by the imaginative name uh...ok (on message boards at least) said this:
Oh, FYI, if I haven't posted about this before, it's called 7-up because it used to have Lithium in it, which would give people temporary highs (which is why it's "Up" but then they took it out later on after finding it had some rather uncool side effects). 7 is Lithium's atomic number, so they called it "7-Up".
It would be fun if that were true! Now it's a couple of years since I was in Secondary School, but isn't Lithium's atomic number three? Nice theory anyway!