Praise then darkness, and creation unfinished

The remark that airships are a sign of alternate history--and possibly that world-splitting generates them the way splitting uranium generates neutrons--has been around for at least a few years. Does anyone remember who first came up with it, or when? My vague memory suggests either Dave Langford or james_nicoll.

Also, can anyone think of an alternate history story with airships earlier than Leiber's "Catch That Zeppelin"?

[Edited to add important qualifier.]