Following on from my recent post about the discussion I started with Eric Schneid regarding the boundary between IA and BA roles, and the ways in which the one can inform the work of the other, I've hit upon the notion that the closer you look at it, the more ill-defined the boundary becomes.
Or rather, the closer in you get the more 'jagged' the separator becomes. This idea is resonant with Mandelbrot's work on fractional dimensions and fractal coastlines in that you can only provide a clear separation if you can define the scale at which you're looking. So you need to use phrases like "in general, this would be done by...".
You can translate this into the rather more prosaic observation that there are many tasks along the edges of the boundary that can, and should, be carried out in collaboration between the two roles rather than being dished out to one or the other. A good example is the definition of technical requirements and business rules. [I'll come back to the 'why' of that at a later date.]
That said, I think there are some definite areas of the project that reside on the business side of the boundary rather than with the user experience/IA role. To my mind these include the interpretation of the brand into a set of experience requirements; the definition of the site's "purpose" or strategy; the definition of commercial rules for the site (aka business rules); the assurance of alignment between business direction and site strategy.
What you should try and avoid wherever possible is to have the user experience/IA role making decisions about what the business needs, or the business strategist interpreting 'what the user wants'. When that starts to happen its probably time for a round-table discussion.
At all costs, I think it's important that these two roles are handled by different individuals. They don't have to be specialists in the area, but I think it's spurious to argue that you can 'swap hats' and make meaningful decisions.
[Exactly what I mean by "interpretation of the brand into a set of experience requirements" is a topic all on its own. As is how you might go about assuring that the site strategy is in alignment with the business strategy.]