Recently, I and Joshua Miller got into a discussion over the importance of Judith Butler over at his blog (bonus, see what books and essays actually changed his mind about something. Which is a great meme that I would like to see reproduced). One of the points he made was that he felt that much of what Judith Butler has been arguing about with precarity is secondary literature on Agamben. I don't think so, and I think this is a great opportunity to clarify the relationship of these two thinkers.
I think it is pretty clear that Agamben has long been an influence on Butler's work. However, I doubt you could call her work on precariousness as secondary literature. First of all, it has none of the form of that (no long textual exegesis, etc.). But the more substantial point is to distinguish Agamben's notion of bare life from Butler's notion of precarious life.
Much of Giorgio Agamben's work is centered around identifying and explicating a series of metaphysical machines that produce modernity. Thus, we have the state of exception in law, we have the anthropological machine in anthropology broadly construed, and we have the providential machine in theology. All of these machines operate in zones of indecision, and all of these machines are fundamentally empty, kenotic. What this means is that each iteration of the decisions of these machines are completely up for grabs each time. There is always a chance that what I do is interpreted as criminal even if it was interpreted as legal last time. The mundane example here is a speeding ticket, where one cop might decide that going 12 over is legal, and the next time the cop might decide it is illegal. The importance is that the machines produce their own justifications in these zones of indecision as if by fiat. Therefore, we all potentially can be seen as killable, we all exist within the nomos of the camp. This is what is meant by bare life-- life that is fundamentally confused between bios and zoe. 
Judith Butler's work on precarious life is very different from Agamben's work on bare life. First of all, there are not these monolithic metaphysical machines populating the work of Butler. Partially this is because Butler is far more interested in the nuances of how certain lives are considered livable and mournable than Agamben is. For Agamben, we all live in the nomos of the camp, and therefore you see Agamben taking up the idea of the archeologist from Foucault, and not the genealogist. To take up the mundane example from before of speeding, you would not see in Agamben any detailed discussions of the types of car your drive, or your race, or the part of town you are in as mattering for how the cop determines to pull you over or not. His metaphysical machines never seem weighted down by history, and their decisions never seem overdetermined by identity. For Butler, the frames by which we determine what gets to count as human, what gets to count as livable life, are all explored with a remarkable specificity. Gender, race, sexuality, ability, etc. all seem to play key roles in figuring out the protocols by which we determine which lives we mourn, and which lives we don't mourn.
The more important distinction, however, is that Butler does not seem to believe in bare life. As she has argued, the tasks you do to reproduce your biological existence (eating and finding food, creating shelter, etc) are all politically and culturally relevant. These are never the tasks of zoe, or of mere existence. This is why Butler talks of the bios of the non-human animal, an insight that I doubt you would ever see from Agamben. Thus, for Butler precariousness is not a condition to be overcome or critiqued, in the way that bare life would be for Agamben, rather precariousness becomes a place to think and organize from. Agamben is never a thinker of vulnerability as enabling, as productive. So, precariousness is not an ahistorical and legal condition, and it is actually something foundational to ontology, ethics, and sociality as such.
 I have so far treated Agamben's earlier work in Homo Sacer and State of Exception as being consistent with his later work in The Kingdom and the Glory. I have done so for some conceptual ease. However, these various works exist in some tension. Peter Gratton makes this argument frequently on his blog, and will be part of his chapter on Agamben in his The State of Sovereignty (SUNY Sometime).