Are there any modern mechanistic theories of motivation? I remember hearing about the old 'drive' theory of motivation in Psych 101, and despite continuing my cognitive science education for another 4 years, that's the last theory of motivation I've heard about. 

Are there any modern theories of motivation at mechanistic level of explanation? 

The only stuff on motivation I find these days are along the lines of Pink's book "Drive", which talks of "purpose" and "mastery" as motivators, but doesn't discuss the actual mechanisms underlying them.
Motivation is a massive topic, and it's difficult for me to know what would count as a 'theory' of motivation as it's currently construed.  For instance, at one level, we might consider motivation to be the processing of incentive salience on perceived stimuli: you see a cheeseburger, something makes you want it, and so you pursue it.  One way of talking about what it means to want this cheeseburger is called incentive salience; see Berridge 1998 for a great review on this topic with a focus on the contentious role of dopamine, and some pretty amazing experiments that pry apart wanting from liking.

Coming from the opposite direction, Carver and Scheier's cybernetic model has proved quite influential in describing motivation, and its hedonic consequences, in terms of progress during goal pursuit.  This goal-centrality dovetails with some of the achievement goal theoretic constructs, one of which (mastery) you mentioned with regard to Pink's book; Andrew Elliot did a nice review here.  For an even more meta computational formulation, Juergen Schmidhuber has for years been proposing an  all-encompassing model that might be grand enough for your tastes, although it is woefully untested by empirical work and, from what I can tell, seems to be considered kind of fringe.

I could go on, but suffice it to say that theories of motivations -- or pieces of theories -- are nearly as numerous as the stars.  The real trick is in unifying them in a coherent system that makes sense at multiple levels of abstraction and isn't just some hodgepodge collection.