On my way back from the signing now. Will give full report upon returning home.
EDIT: Righto, here's his answer in so many words:
"Ah, I've been asked this before. There are a number of theories, but nobody's really sure, since there haven't really been any opportunities to alloy lerasium with atium. You can choose which one to believe. Most require an understanding of realmatic theory to comprehend, which you need to be a Shard or Splinter to even begin to understand."
I interrupted here to ask if Sazed or Hoid would know, and he - of course - RAFO'd.
"What Lerasium is, is essentially a hack for something like your spiritual DNA. It rewrites what your spiritual self is capable of. So, combined with atium, which allows you a glimpse into the vision of everything - past, present, future - the theories say it could do one of two things. It could either create a substance so volatile that it would have world-ending repercussions, or rewrite your "spiritual DNA" (his phrase, not mine) with atium's power. Is that a vague enough answer?"
During the WoK reading, the phrases "Shard-blade" and "Shard-bearer" popped up - my questions on these were RAFO'd as well.
(I won't go into detail about the chapter he read from, since it will apparently be included in the Warbreaker paperback. We can pick it apart after the paperback's release.)