Using user story mapping is a very collaborative way to quickly define what and how the product you intend to deliver is going to get built. During your session(s), team members may raise issues, risks, assumptions and dependencies. Writing them down on stickers is a very nice way to somehow set project boundaries. You probably have already defined your Definition of Done. What you are really doing is writing down an informal project plan, still quite limited but you get valuable inputs from your team!
In “Liftoff: Launching Agile Teams & Projects”, the authors have formalised and have explained how you give teams a sense of purpose by doing up-front project chartering (the process of creating a charter: this is where the value is). So, if user story mapping helps identify some project boundaries, it may be even more beneficial to “liftoff” your project by having a session sooner to look at the goals to achieve and then continue with user story mapping.
http://www.amazon.com/gp/aw/d/097792016X : Liftoff your project helps give a purpose to the teams, align people and provide the context of the project!
How to create a collaborative Definition of Done, see Rally Definition of Done Guide: http://www.rallydev.com/toolkits/definition-done-toolkit