Jameel has a fascinating post up, already linked to by On the Main Line, Godol Hador, and DovBear. Jameel (quoting Makor Rishon) goes back through the history and finds out that it's based on... not much. Check it out, then check S.'s (OtML) where he points out that part of the post may be wrong. DovBear discusses the bonfires, while the Godol merely reflects on the latest holiday to fall.
One thing we do know: Lag B'Omer is still the 33rd day of the Omer.