http://en.wikipedia.org/wiki/Semicolon#Usage
Nevertheless, I also do not take kindly to semicolon usage, because it's so vague. But that does not mean I can claim that it's wrong, without being wrong myself.
In your example, "then" is the conjunctive adverb.
Edit: By the way, I love having grammar discussions on the internet. Let's make a grammar discussion club, so we can really go at it, and maybe try and keep the grammatic national socialism out of hackernews.
Well played -- by way of elaboration, a deft avoidance of Godwin's Law.