> this discussion has pushed me over the edge, it's time I got up to speed -- where would you suggest I start?

It depends: I personally struggled through Anti-Oedipus and Thousand Plateaus, then read Delueze's solo stuff after. It was pretty rough, though: worthwhile, but dense. If you're doing Lacan/Heidegger, I'd imagine you'd be fine. But I'm really super partial to Nietzsche and Philosophy: in the intro, Mike Hardt says it's "the best introduction to both Nietzsche's thought and Deleuze's." My understanding is that it relies a bit too much on The Will To Power for more recent Nietzsche scholarship, but that doesn't necessarily matter... anyway. You should check it out for sure, as it has really interesting relations to _why as well...

TL;DR: A Thousand Plateaus, Nietzsche and Philosophy, in either order.