To my memory l33t speak has its roots in the 'hacker' (or more correctly 'cracker') subculture. First started appearing on warez releases which would advertise BBS numbers, sometimes listing the numbers in plain text (for anyone to access) and sometimes listing as 'elite only' (meaning private access)
As time passed 'l33t' started appearing when refering to those with access to the private numbers. It was used sincerely for a short period, but soon turned into a term solely for mockery. 'l33t speak' followed soon after, which as everyone knows is where numbers replace l3tt3r5. l33t speak was to my memory only ever used in mockery, frequently in scorn either by those with access refering to 'lamers' without access, or vice-versa.